CHAPTER 4 Dosage Form Design: Pharmaceutical and Considerations

After reading this chapter, the student will be able to: 1. List reasons for the incorporation of into various dosage forms 2. Compare and contrast the advantages/disadvantages of various drug dosage forms 3. Describe the information needed in preformulation studies to characterize a drug substance for possible inclusion into a dosage form 4. Describe the mechanisms of drug degradation and provide examples of each

OBJECTIVES 5. Describe the fi ve types of drug instability of concern to the practicing pharmacist 6. Summarize approaches employed to stabilize drugs in pharmaceutical dosage forms 7. Calculate rate reactions for various dosage forms 8. Categorize various pharmaceutical ingredients and

Drug substances are seldom administered alone; prescribe. The general area of study concerned rather they are given as part of a formulation with the formulation, manufacture, stability, and in combination with one or more nonmedicinal effectiveness of pharmaceutical dosage forms is agents that serve varied and specialized pharma- termed pharmaceutics. ceutical functions. Selective use of these nonme- The proper design and formulation of a dicinal agents, referred to as pharmaceutical dosage form requires consideration of the physi- ingredients or excipients, produces dosage forms cal, chemical, and biologic characteristics of all of various types. The pharmaceutical ingredients of the drug substances and pharmaceutical solubilize, suspend, thicken, dilute, emulsify, sta- ingredients to be used in fabricating the prod- bilize, preserve, color, fl avor, and fashion medici- uct. The drug and pharmaceutical materials nal agents into effi cacious and appealing dosage must be compatible with one another to produce forms. Each type of dosage form is unique in a drug product that is stable, effi cacious, attrac- its physical and pharmaceutical characteristics. tive, easy to administer, and safe. The product These varied preparations provide the manufac- should be manufactured with appropriate mea- turing and pharmacist with the sures of quality control and packaged in contain- challenges of formulation and the physician with ers that keep the product stable. The product the choice of drug and delivery system to should be labeled to promote correct use and be 90


stored under conditions that contribute to maxi- 325 mg of found in the common ? mum shelf life. Not possible. Yet compared with many other Methods for the preparation of specifi c types drugs, the of aspirin is formidable (Table 4.1). of dosage forms and drug delivery systems are For example, the dose of ethinyl , 0.05 mg, described in subsequent chapters. This chapter is 1/6,500 the amount of aspirin in an aspirin tab- presents some general considerations regarding let. To put it another way, 6,500 ethinyl estradiol physical , drug product formulation, tablets, each containing 0.05 mg of drug, could be and pharmaceutical ingredients. made from an amount of ethinyl estradiol equal to the amount of aspirin in just one standard tablet. THE NEED FOR DOSAGE FORMS When the dose of the drug is minute, as with ethi- nyl estradiol, dosage forms such as tablets and capsules must be prepared with fi llers or The potent nature and low dosage of most of the diluents so that the dosage unit is large enough to drugs in use today precludes any expectation that pick up with the fi ngertips. the general public could safely obtain the appro- Besides providing the mechanism for the safe priate dose of a drug from the bulk material. Most and convenient delivery of accurate dosage, dos- drug substances are administered in milligram age forms are needed for additional reasons: quantities, much too small to be weighed on any- thing but a sensitive prescription or electronic ana- • To protect the drug substance from the lytical balance. For instance, how could the lay destructive infl uences of atmospheric oxygen person accurately obtain from a bulk supply the or humidity (coated tablets, sealed ampuls)


DRUG USUAL DOSE (MG) CATEGORY Betaxolol HCl 10.00 Antianginal Clotrimoxazole 10.00 Antifungal Methylphenidate HCl 10.00 CNS Medroxyprogesterone acetate 10.00 Progestin Mesoridazine besylate 10.00 Morphine sulfate 10.00 Narcotic analgesic Nifedipine 10.00 Coronary vasodilator Omeprazole 10.00 Antiulcerative Quinapril HCl 10.00 Antihypertensive Chlorazepate dipotassium 7.50 Tranquilizer Buspirone HCl 5.00 Antianxiety Enalapril maleate 5.00 Antihypertensive Hydrocodone 5.00 Narcotic analgesic Prednisolone 5.00 Adrenocortical steroid Albuterol sulfate 4.00 Bronchodilator Chlorpheniramine maleate 4.00 Antihistaminic Felodipine 2.50 Vasodilator Glyburide 2.50 Antidiabetic mesylate 2.00 Antihypertensive Levorphanol tartrate 2.00 Narcotic analgesic Prazosin HCl 2.00 Antihypertensive Risperidone 2.00 Antipsychotic Estropipate 1.25 Estrogen Bumetanide 1.00 Diuretic Clonazepam 1.00 Anticonvulsant Ergoloid mesylates 1.00 Cognitive adjuvant Alprazolam 0.50 Antianxiety 0.50 Gout suppressant Nitroglycerin 0.40 Antianginal Digoxin 0.25 Cardiotonic (maintenance) Levothyroxine 0.10 Thyroid Misoprostol 0.10 Antiulcerative, abortifacient Ethinyl estradiol 0.05 Estrogen


• To protect the drug substance from the destructive infl uence of gastric acid after (enteric-coated tablets) • To conceal the bitter, salty, or offensive taste or odor of a drug substance (capsules, coated tablets, fl avored ) • To provide liquid preparations of substances that are either insoluble or unstable in the desired vehicle (suspensions) • To provide clear liquid dosage forms of sub- stances (syrups, ) • To provide rate-controlled drug action (vari- ous controlled-release tablets, capsules, and suspensions) • To provide optimal drug action from topical administration sites (ointments, , trans- FIGURE 4.1 Various forms of a drug substance marketed by dermal patches, and ophthalmic, , and a Pharmaceutical Company to meet the special requirements nasal preparations) of the patient. • To provide for insertion of a drug into one of the body’s orifi ces (rectal or vaginal ) action), and the age and anticipated condition of • To provide for placement of drugs directly in the patient. the bloodstream or body tissues (injections) If the is intended for systemic use • To provide for optimal drug action through and oral administration is desired, tablets and/or therapy (inhalants and inhalation capsules are usually prepared because they are eas- aerosols) ily handled by the patient and are most convenient in the self-administration of medication. If a drug GENERAL CONSIDERATIONS substance has application in an emergency in which IN DOSAGE FORM DESIGN the patient may be comatose or unable to take oral medication, an injectable form of the medication Before formulating a drug substance into a dosage may also be prepared. Many other examples of form, the desired product type must be determined therapeutic situations affecting dosage form design insofar as possible to establish the framework for could be cited, including motion sickness, nausea, product development. Then, various initial for- and , for which tablets and skin patches mulations of the product are developed and are used for prevention and suppositories and examined for desired features (e.g., drug release injections for treatment. profi le, , clinical effectiveness) and The age of the intended patient also plays a for pilot plant studies and production scale-up. role in dosage form design. For infants and The formulation that best meets the goals for the children younger than 5 years of age, pharma- product is selected to be its master formula. Each ceutical rather than solid forms are pre- batch of product subsequently prepared must ferred for oral administration. These liquids, meet the specifi cations established in the master which are fl avored aqueous solutions, syrups, or formula. suspensions, are usually administered directly There are many different forms into which a into the infant’s or child’s mouth by , spoon, medicinal agent may be placed for the conve- or oral dispenser (Fig. 4.2) or incorporated into nient and effi cacious treatment of disease. Most the child’s . A single liquid pediatric prepa- commonly, a manufacturer prepares a drug sub- ration may be used for infants and children of all stance in several dosage forms and strengths for ages, with the dose of the drug varied by the vol- the effi cacious and convenient treatment of dis- ume administered. When a young patient has a ease (Fig. 4.1). Before a medicinal agent is for- productive cough or is vomiting, gagging, or mulated into one or more dosage forms, among simply rebellious, there may be some question the factors considered are such therapeutic mat- as to how much of the administered is ters as the nature of the illness, the manner in actually swallowed and how much is expecto- which it is treated (locally or through systemic rated. In such instances, injections may be


liquids or may be extemporaneously prepared into an oral liquid by the pharmacist. However, certain tablets and capsules that are designed for controlled release should not be crushed or chewed, because that would interfere with their integrity and intended performance. Many patients, particularly the elderly, take multiple daily. The more distinctive the size, shape, and color of solid dosage forms, the easier is proper identifi cation of the medica- tions. Errors in taking medications among the elderly occur frequently because of their multi- ple drug therapy and impaired eyesight. Dosage forms that allow reduced frequency of adminis- tration without sacrifi ce of effi ciency are particu- larly advantageous. In dealing with the problem of formulating a drug substance into a proper dosage form, research pharmacists employ knowledge gained through experience with other chemically simi- lar drugs and through the proper use of the physical, chemical, biologic, and pharmaceutical sciences. The early stages of any new formula- FIGURE 4.2 Oral dosage devices to assist in measuring tion include studies to collect basic information doses for children. on the physical and chemical characteristics of the drug substance. These basic studies are the required. Infant-size rectal suppositories may preformulation work needed before actual prod- also be employed, although drug absorption uct formulation begins. from the is often erratic. During childhood and even adulthood, a per- PREFORMULATION STUDIES son may have diffi culty swallowing solid dosage Before the formulation of a drug substance into forms, especially uncoated tablets. For this rea- a dosage form, it is essential that it be chemically son, some medications are formulated as chew- and physically characterized. The following pre- able tablets. Many of these tablets are comparable formulation studies (1) and others provide the in texture to an after-dinner mint and break type of information needed to defi ne the nature down into a pleasant-tasting creamy material. of the drug substance. This information provides Newly available tablets dissolve in the mouth in the framework for the drug’s combination with about 10 to 15 seconds; this allows the patient to pharmaceutical ingredients in the fabrication of take a tablet but actually swallow a liquid. a dosage form. Capsules have been found by many to be more easily swallowed than whole tablets. If a Physical Description is moistened in the mouth before it is swallowed, it becomes slippery and readily slides down the It is important to understand the physical descrip- throat with . Also, a teaspoonful of tion of a drug substance prior to dosage form dessert, liquid candy, or placed in the development. Most drug substances in use today mouth and partially swallowed before placing are solid materials, pure chemical compounds of the solid dosage form in the mouth aids in swal- either crystalline or amorphous constitution. The lowing them. Also, if a person has diffi culty swal- purity of the is essential for its lowing a capsule, the contents may be emptied identifi cation and for evaluation of its chemical, into a spoon, mixed with jam, honey, or other physical, and biologic properties. Chemical prop- similar food to mask the taste of the medication erties include structure, form, and reactivity. and swallowed. Medications intended for the Physical properties include such characteristics as elderly are commonly formulated into oral its physical description, particle size, crystalline


structure, melting point, and . Biologic amine hydrobromide is a solid salt of the liquid properties relate to its ability to get to a site of drug scopolamine and is easily pressed into action and elicit a biologic response. tablets. Another approach to formulate liquids Drugs can be used therapeutically as , into solids is by mixing the drug with a solid or liquids, and . Liquid drugs are used to a melted semisolid material, such as a high– much lesser extent than solid drugs; gases, even molecular-weight . The melted less frequently. is poured into hard gelatin capsules to Liquid drugs pose an interesting problem in harden and the capsules sealed. the design of dosage forms and delivery systems. For certain liquid drugs, especially those taken Many liquids are volatile and must be physically orally in large doses or applied topically, their liq- sealed from the atmosphere to prevent evapora- uid nature may have some advantage in therapy. tion loss. Amyl nitrite, for example, is a clear For example, 15-mL doses of mineral may be yellowish liquid that is volatile even at low tem- administered conveniently as such. Also, the liq- peratures and is also highly fl ammable. It is kept uid nature of undecylenic acid certainly does not for medicinal purposes in small sealed glass cyl- hinder but rather enhances its use topically in the inders wrapped with gauze or another suitable treatment of fungus of the skin. How- material. When amyl nitrite is administered, ever, for the most part, pharmacists prefer solid the glass is broken between the fi ngertips, and materials in formulation work because they can the liquid wets the gauze covering, producing easily form them into tablets and capsules. vapors that are inhaled by the patient requiring Formulation and stability diffi culties arise less vasodilation. Propylhexedrine is another volatile frequently with solid dosage forms than with liq- liquid that must be contained in a closed system. uid preparations, and for this reason many new This drug is used as a nasal inhalant for its vaso- drugs fi rst reach the market as tablets or dry- constrictor action. A cylindrical roll of fi brous fi lled capsules. Later, when the pharmaceutical material is impregnated with propylhexedrine, problems are resolved, a liquid form of the same and the saturated cylinder is placed in a suitable, drug may be marketed. This procedure is doubly usually plastic, sealed nasal . The inhaler’s advantageous, because for the most part physi- cap must be securely tightened each time it is cians and patients alike prefer small, generally used. Even then, the inhaler maintains its effec- tasteless, accurately dosed tablets or capsules to tiveness for only a limited time because of the the analogous liquid forms. Therefore, market- volatility of the drug. ing a drug in solid form fi rst is more practical for Another problem associated with liquid drugs the manufacturer and suits most patients. It is is that those intended for oral administration estimated that tablets and capsules constitute cannot generally be formulated into tablet form, the dosage form dispensed 70% of the time by the most popular form of oral medication, community pharmacists, with tablets dispensed without chemical modifi cation. An exception to twice as frequently as capsules. this is the liquid drug nitroglycerin, which is for- mulated into sublingual tablets that disintegrate Microscopic Examination within seconds after placement under the tongue. However, because the drug is volatile, it has a Microscopic examination of the raw drug sub- tendency to escape from the tablets during stor- stance is an important step in preformulation age, and it is critical that the tablets be stored in work. It gives an indication of particle size and a tightly sealed glass container. For the most part, size range of the raw material along with the when a liquid drug is to be administered orally crystal structure. Photomicrographs of the initial and a solid dosage form is desired, one of two and subsequent batch lots of the drug substance approaches is used. First, the liquid substance can provide important information in case of may be sealed in a soft gelatin capsule. Vitamins problems in formulation processing attributable A, D, and E, cyclosporin (Neoral, Sandimmune), to changes in particle or crystal characteristics of and ergoloid mesylates (Hydergine LC) are liq- the drug. During some processing procedures, uids commercially available in capsule form. Sec- the solid drug must fl ow freely and not ond, the liquid drug may be developed into a become entangled. Spherical and oval powders solid ester or salt form that will be suitable for fl ow more easily than needle-shaped powders tablets or drug capsules. For instance, scopol- and make processing easier.


Heat of Vaporization to cyclophosphamide, etoposide, cisplatin, and 5-fl uorouracil, as illustrated in Physical Phar- The use of vapor pressure is important in the macy Capsule 4.1, Heat of Vaporization. operation of implantable pumps delivering med- ications as well as in aerosol dosage forms. Another application is the use of nasal inhalants Melting Point Depression (propylhexedrine with menthol and lavender oil- A characteristic of a pure substance is a defi ned Benzedrex) for treating nasal congestion. In this melting point or melting range. If not pure, the latter dosage form, the quantity of drug required substance will exhibit a change in melting point. for effectiveness and a reasonable estimate of This phenomenon is commonly used to determine time of usefulness can be determined. Also, in the purity of a drug substance and in some cases the case of spills in inaccessible places, the time the compatibility of various substances before to evaporation of a substance can also be calcu- inclusion in the same dosage form. This character- lated. Some volatile drugs can even migrate istic is further described in Physical Pharmacy within a tablet dosage form so the distribution Capsule 4.2, Melting Point Depression. may not be uniform any longer. This may have an impact in tablets that are scored for dosing The Rule where the drug in one portion may be higher or lower than in the other portion. Phase diagrams are often constructed to provide Exposure of personnel to hazardous drugs a visual picture of the existence and extent of the due to handling, spilling, or aerosolizing of drugs presence of solid and liquid phases in binary, ter- that may vaporize (oncology agents) is another nary, and other . Phase diagrams are nor- application as the increase in mobility of the haz- mally two-component (binary) representations, ardous drug molecules may be related to tem- as shown in Physical Pharmacy Capsule 4.3, The perature of the environment. Some drugs, such Phase Rule, but can also be three-component as carmustine, experience greater vapor pres- representations, as shown in Physical Pharmacy sures with increased temperature as compared Capsule 4.4, Triangular Phase Diagram.

PHYSICAL PHARMACY CAPSULE 4.1 Heat of Vaporization

The amount of heat absorbed when 1 g of a liquid vaporizes is known as the heat of vaporization of that liquid and is measured in calories. The heat of vaporization of water at 100°C is 540 cal/g or about 9.720 cal/mole. This is the same quantity of heat energy that is released when 1 g of steam condenses to water at 100°C. This energy exchange is important in processes like steam sterilization as it is this energy transfer that results in death of . The movement of molecules varies with temperature. In liquids, this results in a tendency of the mol- ecules to escape the liquid environment into a gaseous environment and possibly loss of the liquid. In the case of solids that sublime, the movement of the molecules is from the solid state to the vapor state. As an example, if one looks at an older bottle containing aspirin, there may be crystals of aspirin on the inside walls of the container. With ibuprofen, the walls of the container may become cloudy as the ibuprofen sublimes. The use of vapor pressure is important in the operation of implantable pumps delivering medications as well as in aerosol dosage forms. Exposure of personnel to hazardous drugs due to handling, spilling, or aerosolizing of drugs that may vaporize (oncology agents) is another application as the increase in mobility of the hazardous drug molecules may be related to temperature of the environment. Some drugs, such as carmustine, experience greater vapor pressures with increased temperature as compared to cyclo- phosphamide, etoposide, cisplatin and 5-fl uorouracil, as illustrated in the table below. Particle size affects vapor pressure; the smaller the particle size, the greater the vapor pressure. This demonstrates the impor- tance of personnel protection with working with micronized hazardous powders. The time to evaporation of a substance can also be calculated.



The variation of vapor pressure with temperature is described by the form of the Clausius–Clapeyron equation, as follows: ΔH dP ln = vap dT RT2 δ assuming that Hvap is constant, integration of the equation gives: −Δ H log P =+vap constant 2.303 RT A plot of the log of the vapor pressure versus 1/T should be linear and the slope will equal Δ − Hvap/2.303R from which the enthalpy of vaporization can be calculated. With data obtained from Kiffmeyer TK, Kube C, Opiolka S, et al. Pharm J 2002;268:331, the following table can be constructed:

MEASURED VAPOR PRESSURE (Pa) COMPOUND 20°C 40°C Carmustine 0.019 0.530 Cisplatin 0.0018 0.0031 Cyclophosphamide 0.0033 0.0090 Etoposide 0.0026 0.0038 Fluorouracil 0.0014 0.0039

PHYSICAL PHARMACY CAPSULE 4.2 Melting Point Depression

The melting point, or freezing point, of a pure crystalline solid is defi ned as the temperature at which the pure liquid and solid exist in equilibrium. Drugs with a low melting point may soften during a processing step in which heat is generated, such as particle size reduction, compression, sintering, and so on. Also, the melting point or range of a drug can be used as an indicator of purity of chemical substances (a pure substance is ordinarily characterized by a very sharp melting peak). An altered peak or a peak at a different temperature may indicate an adulterated or impure drug. This is explained as follows. The latent heat of fusion is the quantity of heat absorbed when 1 g of a solid melts; the molar heat of Δ fusion ( Hf) is the quantity of heat absorbed when 1 mole of a solid melts. High–melting-point substances have high heat of fusion, and low-melting-point substances have low heat of fusion. These characteristics are related to the types of bonding in the specifi c substance. For example, ionic materials have high heats of fusion (NaCl melts at 801°C with a heat of fusion of 124 cal/g), and those with weaker van der Waals forces have low heats of fusion (paraffi n melts at 52°C with a heat of fusion of 35.1 cal/g). Ice, with weaker hydrogen bonding, has a melting point of 0°C and a heat of fusion of 80 cal/g. The addition of a second component to a pure compound (A), resulting in a mixture, will result in a melt- ing point that is lower than that of the pure compound. The degree to which the melting point is lowered is

proportional to the mole fraction (NA) of the second component that is added. This can be expressed thus: 2.303 RT T Δ=T 0 log N Δ A Hf

where Δ Hf is the molar heat of fusion, T is the absolute equilibrium temperature,

T0 is the melting point of pure A, and R is the constant.



Two noteworthy things contribute to the extent of lowering of the melting point: 1. Evident from this relationship is the inverse proportion between the melting point and the heat of fusion. When a second ingredient is added to a compound with a low molar heat of fusion, a large lowering of the melting point is observed; substances with a high molar heat of fusion will show little change in melting point with the addition of a second component. 2. The extent of lowering of the melting point is also related to the melting point itself. Compounds with low melting points are affected to a greater extent than compounds with high melting points upon the addition of a second component (i.e., low–melting-point compounds will result in a greater lowering of the melting point than those with high melting points).


A phase diagram, or temperature-composition diagram, represents the melting point as a function of composi- tion of two or three component systems. The fi gure is an example of such a representation for a two-component mixture. This phase diagram depicts a two-component mix- ture in which the components are completely miscible in the molten state and no solid or addition com- pound is formed in the solid state. As is evident, starting from the extremes of either pure component A or pure component B, as the second component is added, the melting point of the pure component decreases. There is a point on this phase diagram at which a minimum melting point occurs (i.e., the eutectic point). As is evident, four regions, or phases, in this diagram, represent the following: I Solid A + solid B II Solid A + melt III Solid B + melt IV Melt Each phase is a homogenous part of the system, physically separated by distinct boundaries. A description of the conditions under which these phases can exist is called the Phase Rule, which can be presented thus: F = C − P + X where F is the number of degrees of freedom, C is the number of components, P is the number of phases, and X is a variable dependent upon selected considerations of the phase diagram (1, 2, or 3). C describes the minimum number of chemical components to be specifi ed to defi ne the phases. F is the number of independent variables that must be specifi ed to defi ne the complete system (e.g., temperature, pressure, concentration).



EXAMPLE 1 In a mixture of menthol and thymol, a phase diagram similar to that illustrated can be obtained. To describe the number of degrees of freedom in the part of the graph moving from the curved line starting at pure A, progressing downward to the eutectic point, and then following an increasing melting point to pure B, it is evident from this presentation that either temperature or composition will describe this system, since it is assumed in this instance that pressure is constant. Therefore, the number of degrees of freedom to describe this portion of the phase diagram is given thus: F = 2 − 2 + 1 = 1 In other words, along this line either temperature or composition will describe the system.

EXAMPLE 2 When in the area of a single phase of the diagram, such as the melt (IV), the system can be described thus: F = 2 − 1 + 1 = 2 In this portion of the phase diagram, two factors, temperature and composition, can be varied without a change in the number of phases in the system.

EXAMPLE 3 At the eutectic point, F = 2 − 3 + 1 = 0 and any change in the concentration or temperature may cause disappearance of one of the two solid phases or the liquid phase. Phase diagrams are valuable for interpreting interactions between two or more components, relating not only to melting point depression and possible liquefaction at room temperature but also the forma- tion of solid solutions, coprecipitates, and other solid-state interactions.

PHYSICAL PHARMACY CAPSULE 4.4 Triangular (Three-component) Phase Diagram

A three-component phase diagram has four degrees of freedom: F = 3 − 1 + 2 = 4. In this case, temperature and pressure are two of the conditions and the concentrations of two of the three components make up the rest. Only two concentrations are required because the third will be the difference between 100% and the sum of the other two components. These systems are used for determining /solubility, coacervation regions, -forming regions for multicomponent mixtures, etc. To read a 3-phase diagram, each of the three corners of the triangle represent 100% by weight of one of the components (A, B, C) and 0% by weight of the other two (A, B, C). The lines joining the corner points forming the triangle each represent two component mixtures of the three possible combinations (AB, BC, and CA). If two of the components are known, the third is known by difference. Any combination of the three components is described by a single point on the diagram. Combining different proportions of the three components and observing for an end point (solubility, gel-formation, haziness, etc.), the phase differences can be visualized, as follows.



The following is a stack of four separate pseudoternary phase diagrams for a quaternary system composed of Brij 96, glycerin, , Mineral oil and water. The Brij 96:glycerin ratio is noted on the diagram and is considered one of three com- ponents. The shaded regions represent gel systems while the clear regions represent fl uid systems.

In addition to observing the phase changes in Water a single plane, the use of stacked ternary phase 3:1 diagrams enables one to visualize the change 1:1 using different ratios of one of the components 1:3 (in this case, the Brij 96:glycerin ratios). Construc- Brij 96 + Glycerin tions like this enable a pharmaceutical scientist to select the best ratios and combinations of components for a formulation.

Particle Size tent uniformity in solid dosage forms depends to a large degree on particle size and the equal Certain physical and chemical properties of distribution of the active ingredient throughout drug substances, including dissolution rate, bio- the formulation. Particle size is discussed fur- availability, content uniformity, taste, texture, ther in Chapter 6. Figure 4.3 shows a particle color, and stability, are affected by the particle size analyzer. size distribution. In addition, fl ow characteris- tics and rates, among other prop- Polymorphism erties, are important factors related to particle size. It is essential to establish as early as possi- An important factor on formulation is the crystal ble how the particle size of the drug substance or amorphous form of the drug substance. may affect formulation and effi cacy. Of special Polymorphic forms usually exhibit different interest is the effect of particle size on absorp- physicochemical properties, including melting tion. Particle size signifi cantly infl uences the point and solubility. Polymorphic forms in drugs oral absorption profi les of certain drugs, includ- are relatively common. It has been estimated ing griseofulvin, nitrofurantoin, , that at least one third of all organic compounds and procaine penicillin. Also, satisfactory con- exhibit polymorphism. In addition to polymorphic forms, compounds may occur in noncrystalline or amorphous forms. The energy required for a molecule of drug to escape from a crystal is much greater than is required to escape from an amorphous . Therefore, the amorphous form of a compound is always more soluble than a corresponding crystal form. Evaluation of crystal structure, polymorphism, and solvate form is an important preformulation activity. The changes in crystal characteristics can infl uence bioavailability and chemical and physical stability and can have important impli- cations in dosage form process functions. For FIGURE 4.3 Mastersizer 2000E particle size analyzer. example, it can be a signifi cant factor relating to (Courtesy of Malvern Instruments Ltd.) tablet formation because of fl ow and compaction


behaviors, among others. Various techniques are A drug’s solubility is usually determined by used to determine crystal properties. The most the equilibrium solubility method, by which an widely used methods are hot stage microscopy, excess of the drug is placed in a and thermal analysis, infrared spectroscopy, and shaken at a constant temperature over a long X-ray diffraction. period until equilibrium is obtained. Chemical analysis of the drug content in solution is per- Solubility formed to determine degree of solubility. An important physicochemical property of a drug substance is solubility, especially aqueous Solubility and Particle Size system solubility. A drug must possess some Although solubility is normally considered a aqueous solubility for therapeutic effi cacy. For a physicochemical constant, small increases in drug to enter the systemic circulation and exert a solubility can be accomplished by particle size therapeutic effect, it must fi rst be in solution. reduction as described in the Physical Pharmacy Relatively insoluble compounds often exhibit Capsule 4.5, Solubility and Particle Size. incomplete or erratic absorption. If the solubility of the drug substance is less than desirable, con- Solubility and pH sideration must be given to improve its solubil- ity. The methods to accomplish this depend on Another technique, if the drug is to be formu- the chemical nature of the drug and the type of lated into a liquid product, is adjustment of drug product under consideration. Chemical the pH of the solvent to enhance solubility. modifi cation of the drug into salt or ester forms However, for many drug substances pH adjust- is frequently used to increase solubility. ment is not an effective means of improving

PHYSICAL PHARMACY CAPSULE 4.5 Solubility and Particle Size

The particle size and surface area of a drug exposed to a medium can affect actual solubility within reason, for example, in the following relationship: S 2Vγ log = S0 2.303 RTr where S is the solubility of the small particles,

S0 is the solubility of the large particles, γ is the , V is the molar volume, R is the gas constant, T is the absolute temperature, and r is the radius of the small particles. The equation can be used to estimate the decrease in particle size required to increase solubility. For

example, a desired increase in solubility of 5% would require an increase in the S/S0 ratio to 1.05; that is, the left term in the equation would become log 1.05. If a powder has a surface tension of 125 dynes per centimeter, molar volume of 45 cm3, and temperature of 27°C, what is the particle size required to obtain the 5% increase in solubility? (2) (125) (45) log1.05 = (2.303) (8.314 × 107) (300)r r = 9.238 × 10-6 cm or 0.09238m A number of factors are involved in actual solubility enhancement, and this is only an introduction to the general effects of particle size reduction.


solubility. Weak acidic or basic drugs may aqueous solubility. A review of pH is provided require extremes in pH that are outside in Physical Pharmacy Capsule 4.6, Principles accepted physiologic limits or that may cause of pH. The effect of pH on solubility is illus- stability problems with formulation ingredients. trated in Physical Pharmacy Capsule 4.7, Sol- Adjustment of pH usually has little effect on ubility and pH. the solubility of substances other than electro- In recent years, more and more physico- lytes. In many cases, it is desirable to use cosol- chemical information on drugs is being made vents or other techniques such as complexation, available to pharmacists in routinely used refer- micronization, or solid to improve ence books. This type of information is important


pH is a critical variable in pharmaceutics, and a basic understanding of its principles and measurement are important. Let’s begin with a defi nition of the term pH. The p comes from the word power. The H, of course, is the symbol for hydrogen. Together, the term pH means the hydrogen ion exponent. The pH of a substance is a measure of its acidity, just as a degree is a measure of temperature. A spe- cifi c pH value tells the exact acidity. Rather than stating general ideas, such as cherry syrup is acidic or the water is hot, a specifi c pH value gives the same relative point of reference, thus providing more exact communication. “The cherry juice has a pH of 3.5” or “the water is at 80°C” provides an exact common language. pH is defi ned in terms of the hydrogen ion activity:

−pH pH = -log10 aH+ or 10 = aH+ pH equals the negative logarithm of the hydrogen ion activity, or the activity of the hydrogen ion is 10 raised to the exponent −pH. The latter expression renders the use of the p exponent more obvious. The activity is the effective concentration of the hydrogen ion in solution. The difference between effective and actual concentration decreases as one moves toward more dilute solutions, in which ionic interaction becomes progressively less important. Normally, reference is made to the hydrogen ion when reference should be made to the hydronium ion + (H3O ). It is a of convenience and brevity that only the hydrogen ion is mentioned, even though it is normally in its solvated form:

+ + H + H2O = H3O The complexing of the hydrogen ion by water affects activity and applies to other ions, which partially complex or establish an equilibrium with the hydrogen ion. In other words, equilibrium such as =++− H23 CO H HCO3 − HCHO= H+CHO+ 23 2 23 2 complexes the hydrogen ion so that it is not sensed by the pH measuring system. This is why an acid-base titration is performed if the total concentration of acid (H+) is needed. These effects on hydrogen ion activ- ity are obvious, but other more subtle effects are involved in the correlation of activity and concentration. + The activity of the hydrogen ion can be defi ned by its relation to concentration (CH , molality) and the + activity coeffi cient fH :

+ aH = fH + CH+ If the activity coeffi cient is unity, activity is equal to concentration. This is nearly the case in dilute solu- tions, whose ionic strength is low. Since the objective of most pH measurements is to fi nd a stable and reproducible reading that can be correlated to the results of some process, it is important to know what infl uences the activity coeffi cient and therefore the pH measurement.



The factors that affect the activity coeffi cient are the temperature (T), the ionic strength (μ), the dielec- ε tric constant ( ), the ion charge (Zi), the size of the ion in angstroms (Å), and the density of the solvent (d). All of these factors are characteristics of the solution that relate the activity to the concentration by two main effects: the salt effect and the medium effect; the latter relates the infl uence that the solvent can have on the hydrogen ion activity. Thus, hydrogen activity is related to concentration through a salt effect and a solvent effect. Because of these infl uences, a sample pH value cannot be extrapolated to another tempera- ture or dilution. If the pH value of a particular solution is known at 40°C, it is not automatically known at 25°C.

THE pH SCALE In pure water, hydrogen and hydroxyl ion concentrations are equal at 10−7 M at 25°C. This is a neutral solu- tion. Since most samples encountered have less than 1 M H+ or OH−, the extremes of pH 0 for acids and pH 14 for bases are established. Of course, with strong acids or bases, pH values below 0 and above 14 are possible but infrequently measured.

MEASUREMENT OF pH The activity of the hydrogen ion in solution is measured with a glass electrode, a reference electrode, and a pH meter.

COMBINATION ELECTRODES A combination electrode is a combination of the glass and reference electrodes into a single probe. The main advantage in using a combination electrode is with the measurement of small volume samples or samples in limited-access containers.


pH is one of the most important factors in the formulation process. Two areas of critical importance are the effects of pH on solubility and stability. The effect of pH on solubility is critical in the formulation of liquid dosage forms, from oral and topical solutions to intravenous solutions and admixtures. The solubility of a weak acid or base is often pH dependent. The total quantity of a monoprotic weak acid (HA) in solution at a specifi c pH is the sum of the concentrations of both the free acid and salt (A−) forms. If excess drug is present, the quantity of free acid in solution is maximized and constant because of its saturation solubility. As the pH of the solution increases, the quantity of drug in solution increases because the water-soluble ionizable salt is formed. The expression is

Ka HA ↔ H+ + A–

where Ka is the dissociation constant.

There may be a certain pH level reached where the total solubility (ST) of the drug solution is saturated

with respect to both the salt and acid forms of the drug, that is, the pHmax. The solution can be saturated with respect to the salt at pH values higher than this, but not with respect to the acid. Also, at pH values



less than this, the solution can be saturated with respect to the acid but not to the salt. This is illustrated in the accompanying fi gure. To calculate the total quantity of drug that can be maintained in solution at a selected pH, either of two equations can be used, depending on whether the prod-

uct is to be in a pH region above or below the pHmax. The

following equation is used when below the pHmax: ⎛⎞ + Ka S =S⎜⎟ 1 + (Equation 1) Ta⎝⎠[H ]

The next equation is used when above the pHmax: ⎛⎞ + Ka S = S'⎜⎟ 1 + (Equation 2) Ta⎝⎠[H ]


Sa is the saturation solubility of the free acid and ′ Sa is the saturation solubility of the salt form.

EXAMPLE A pharmacist prepares a 3.0% solution of an antibiotic as an ophthalmic solution and dispenses it to a patient. A few days later the patient returns the eye drops to the pharmacist because the product contains a precipitate. The pharmacist, checking the pH of the solution and fi nding it to be 6.0, reasons that the problem may be pH related. The physicochemical information of interest on the antibiotic includes the following: Molecular weight 285 (salt) 263 (free acid) 3.0% solution of the drug 0.1053 M solution

Acid form solubility (Sa) 3.1 mg/mL (0.0118 M) –6 Ka 5.86 × 10 Using Equation 1, the pharmacist calculates the quantity of the antibiotic in solution at a pH of 6.0 (Note: pH of 6.0 = [H+] of 1 × 10–6)

ST = 0.0118[1+] = 0.0809 molar From this the pharmacist knows that at a pH of 6.0, a 0.0809-M solution can be prepared. However, the concentration that was to be prepared was a 0.1053-M solution; consequently, the drug will not be in solution at that pH. The pH may have been all right initially but shifted to a lower pH over time, resulting in precipitation of the drug. The question is at what pH (hydrogen ion concentration) the drug

will remain in solution. This can be calculated using the same equation and information. The ST value is 0.1053 M.

⎡⎤5.86× 10−6 0.1053=+ 0.0118⎢⎥ 1 ⎣⎦[H+ ] [H+− ]=× 7.333 107 , or a pH of 6.135 The pharmacist prepares a solution of the antibiotic, adjusting the pH to above about 6.2, using a suitable buffer system, and dispenses the solution to the patient—with positive results.



An interesting phenomenon concerns the close relationship of pH to solubility. At a pH of 6.0, only a 0.0809-M solution could be prepared, but at a pH of 6.13 a 0.1053-M solution could be prepared. In other words, a difference of 0.13 pH units resulted in 01053-0.0809 = 30.1% 0.0809 more drug going into solution at the higher pH than at the lower pH. In other words, a very small change in pH resulted in about 30% more drug going into solution. According to the fi gure, the slope of the curve would be very steep for this example drug, and a small change in pH (x-axis) results in a large change in solubility (y-axis). From this, it can be reasoned that if one observes the pH–solubility profi le of a drug, it is possible to predict the magnitude of the pH change on its solubility. In recent years, more and more physicochemical information on drugs is being made available to pharmacists in routinely used reference books. This type of information is important for pharmacists in different types of practice, especially those who compound and do pharmacokinetic .

for pharmacists in different types of practice, in the form of fi ne particles with a large surface especially those involved in compounding and area. pharmacokinetic monitoring. The dissolution rates of chemical compounds are determined by two methods: the constant- Dissolution surface method, which provides the intrinsic dissolution rate of the agent, and particulate Variations in the biologic activity of a drug sub- dissolution, in which a of the agent stance may be brought about by the rate at is added to a fi xed amount of solvent without which it becomes available to the organism. In exact control of surface area. many instances, dissolution rate, or the time it The constant-surface method uses a com- takes for the drug to dissolve in the fl uids at pressed disc of known area. This method the absorption site, is the rate-limiting step in eliminates surface area and surface electrical absorption. This is true for drugs administered charges as dissolution variables. The dissolution orally in solid forms such as tablets, capsules, rate obtained by this method, the intrinsic or suspensions, and for those administered dissolution rate, is characteristic of each solid intramuscularly. When the dissolution rate is compound and a given solvent in the fi xed the rate-limiting step, anything that affects it experimental conditions. The value is expressed will also affect absorption. Consequently, dis- as milligrams dissolved per minute per centime- solution rate can affect the onset, intensity, and ters squared. It has been suggested that this duration of response and control the overall value is useful in predicting probable absorption bioavailability of the drug from the dosage problems due to dissolution rate. In particulate form, as discussed in the previous chapter. dissolution, a weighed amount of powdered The dissolution rate of drugs may be increased sample is added to the dissolution medium in a by decreasing the drug’s particle size. It may also constant agitation system. This method is fre- be increased by increasing its solubility in the quently used to study the infl uence of particle diffusion layer. The most effective means of size, surface area, and excipients upon the active obtaining higher dissolution rates is to use a agent. Occasionally, the surface properties of highly water-soluble salt of the parent substance. the drug produce an inverse relationship of par- Although a soluble salt of a weak acid will pre- ticle size to dissolution. In these instances, sur- cipitate as the free acid in the bulk phase of an face charge and/or agglomeration results in the acidic solution, such as gastric fl uid, it will do so reduced particle size form of the drug presenting


a lower effective surface area to the solvent due Membrane Permeability to incomplete wetting or agglomeration. Fick’s laws describe the relationship of diffusion and Modern preformulation studies include an dissolution of the active drug in the dosage form early assessment of passage of drug molecules and when administered in the body, as shown in across biologic membranes. To produce a bio- Physical Pharmacy Capsule 4.8, Fick’s Laws of logic response, the drug molecule must fi rst Diffusion and the Noyes–Whitney Equation. cross a biologic membrane. The biologic mem- Early formulation studies should include the brane acts as a barrier to most drugs and effects of pharmaceutical ingredients on the dis- permits the absorption of lipid-soluble sub- solution characteristics of the drug substance. stances by passive diffusion, while lipid-insoluble

PHYSICAL PHARMACY CAPSULE 4.8 Fick’s Laws of Diffusion and the Noyes-Whitney Equation

All drugs must diffuse through various barriers when administered to the body. For example, some drugs must diffuse through the skin, gastric mucosa, or some other barrier to gain access to the interior of the body. Parenteral drugs must diffuse through muscle, connective tissue, and so on, to get to the site of action; even intravenous drugs must diffuse from the blood to the site of action. Drugs must also diffuse through various barriers for and excretion. Considering all the diffusion processes that occur in the body (passive, active, and facilitated), it is not surprising that the laws governing diffusion are important to drug delivery systems. In fact, diffusion is important not only in the body but also in some quality control procedures used to determine batch-to- batch uniformity of products (dissolution test for tablets based on the Noyes-Whitney equation, which can be derived from Fick’s law). When individual molecules move within a substance, diffusion is said to occur. This may occur as the result of a concentration gradient or by random molecular motion. Probably the most widely used laws of diffusion are known as Fick’s fi rst and second laws. Fick’s fi rst law involving steady-state diffusion (where dc/dx does not change) is derived from the following expres- sion for the quantity of material (M) fl owing through a cross section of a barrier (S) in unit time (t) expressed as the fl ux (J): J = dM/(Sdt) Under a concentration gradient (dc/dx), Fick’s fi rst law can be expressed thus:

J = D[(C1-C2)/h] or J = -D(dC/dx) where J is the fl ux of a component across a plane of unit area,

C1 and C2 are the concentrations in the donor and receptor compartments, h is the membrane thickness, and D is the diffusion coeffi cient (or diffusivity). The sign is negative, denoting that the fl ux is in the direction of decreasing concentration. The units of J are grams per square centimeter; C, grams per cubic centimeter; M, grams or moles; S, square centimeters; x, centimeters; and D, square centimeters per second. D is appropriately called a diffusion coeffi cient, not a diffusion constant, as it is subject to change. D may change in value with increased concentrations. Also, D can be affected by temperature, pressure, solvent properties, and the chemical nature of the drug itself. To study the rate of change of the drug in the system, one needs an expression that relates the change in concentration with time at a defi nite location in place of the mass of drug diffusing across a unit area of barrier in unit time; this expression is known as Fick’s second law. This law can be summarized as stating that the change in concentration in a particular place with time is proportional to the change in concentration gradient at that particular place in the system.



In summary, Fick’s fi rst law relates to a steady-state fl ow, whereas Fick’s second law relates to a change in concentration of drug with time, at any distance, or an unsteady state of fl ow. The diffusion coeffi cients (D × 10−6) of various compounds in water (25°C) and other media have been determined as follows: , 12.5 cm2 per second; glycine, 10.6 cm2 per second; sodium lauryl sulfate, 6.2 cm2 per second; glucose, 6.8 cm2 per second. The concentration of drug in the membrane can be calculated using the partition coeffi cient (K) and the concentration in the donor and receptor compartments.

K = (C1/Cd) = (C2/Cr) where

3 C1 and Cd are the concentrations in the donor compartment (g/cm ) 3 and C2 and Cr are the concentrations in the receptor compartment (g/cm ). K is the partition coeffi cient of the drug between the solution and the membrane. It can be estimated using the oil solubility of the drug versus the water solubility of the drug. Usually, the higher the partition coeffi cient, the more the drug will be soluble in a lipophilic substance. We can now write the expression: − dM/dt = [DSK(Cd Cr)]/h or in sink conditions,

dM/dt = DSKCd /h = PSCd The permeability coeffi cient (centimeters per second) can be obtained by rearranging to:

P = DK /h

EXAMPLE 1 A drug passing through a 1-mm-thick membrane has a diffusion coeffi cient of 4.23 × 10−7 cm2 per second and an oil–water partition coeffi cient of 2.03. The radius of the area exposed to the solution is 2 cm, and the concentration of the drug in the donor compartment is 0.5 mg/mL. Calculate the permeability and the diffusion rate of the drug. h = 1 mm = 0.1 cm D = 4.23 × 10−7 cm2/second K = 2.03 r = 2 cm, S = π(2 cm)2 = 12.57 cm2 Cd = 0.5 mg/mL P = [(4.23 × 10−7 cm2/second) (2.03)]/0.1 cm = 8.59 × 10−6 cm/second dM/dt = (8.59 × 10−6 cm/second) (12.57 cm2)(0.5 mg/mL) = 5.40 × 10−5 mg/second (5.40 × 10−5 mg/second)(3600 second/hour) = 0.19 mg/hour In the dissolution of particles of drug, the dissolved molecules diffuse away from the individual particle body. An expression to describe this, derived from Fick’s equations, is known as the Noyes and Whitney expression, proposed in 1897. It can be written as follows: dC/dt = (DS/Vh) (Cs-C)

where C is the concentration of drug dissolved at time t, D is the diffusion coeffi cient of the solute in solution, S is the surface area of the exposed solid,



V is the volume of solution, h is the thickness of the diffusion layer, Cs is the saturation solubility of the drug, and C is the concentration of solute in the bulk phase at a specifi c time, t. It is common practice to use sink conditions in which C does not exceed about 20% of the solubility of the drug being investigated. Under these conditions, the expression simplifi es to dC/dt = (DSCs/Vh) and incorporating the volume of solution (V), the thickness of the diffusion layer (h), and the diffusivity coeffi cient (D) into a coeffi cient k (to take into account the various factors in the system), the expression becomes dC/dt = kSCs

As the factors are held constant, it becomes apparent that the dissolution rate of a drug can be propor- tional to the surface area exposed to the dissolution medium. A number of other expressions have been derived for specifi c application to various situations and conditions. These relationships expressed as Fick’s fi rst and seconwd laws and the Noyes-Whitney equation have great importance and relevance in pharmaceutical systems.

EXAMPLE 2 The following information was obtained using the USP 32–NF 27 dissolution apparatus I. The drug is soluble at 1 g in 3 mL of water, so sink conditions were maintained; the surface area of the tablet exposed was 1.5 cm2 (obtained by placing the tablet in a special holder exposing only one side to the dissolution medium); and the dosage form studied was a 16-mg sustained-release tablet; the release pattern should be zero order. What is the rate of release of drug?


0.0 0.0 16 0.5 1.0 12 1.0 1.9 2.0 4.1 8

4.0 8.0 4 6.0 11.8

Drug Concentration (mg/900mL) 0 2 468 8.0 15.9 Time (hr)

In this problem, since the surface area (S) was maintained constant at 1.5 cm2 and the solubility (Cs) of the drug is constant at 1 g in 3 mL of water, the plot of concentration versus time (t) yields a slope with a

value of kSCs, or k2, expressing the rate of release of the drug as dC/dt = kSCs = Δ Δ = − − The slope of the line y/ x (y2 y1)/(x2 x1) = (15.9 mg − 0 mg)/(8.0 h − 0 h) = 15.9/8 = 1.99 mg/h Therefore, the rate of release of the sustained-release preparation is 1.99, or approximately 2 mg per hour. From this, the quantity of drug released at any time (t) can be calculated.


substances can diffuse across the barrier only Partition Coeffi cient with considerable diffi culty if at all. The inter- The use of the partition coeffi cient is described relationship of the dissociation constant, lipid in some detail in Physical Pharmacy Capsule 4.9, solubility, and pH at the absorption site with the Partition Coeffi cient. Inherent in this procedure absorption characteristics of various drugs are is the selection of appropriate extraction sol- the basis of the pH partition theory. vents, drug stability, use of salting-out additives, Data obtained from the basic physicochemi- and environmental concerns. The octanol–water cal studies, specifi cally, pKa, solubility, and dis- partition coeffi cient is commonly used in formu- solution rate, provide an indication of absorption. lation development. Following the illustrations To enhance these data, a technique using the provided earlier, it is defi ned as everted intestinal sac may be used to evaluate absorption characteristics of drug substances. In (Conc. of drug in octanol) this method, a piece of intestine is removed from P= an intact animal, is everted, and is fi lled with a (Conc. of drug in water) solution of the drug substance, and the degree and rate of passage of the drug through the mem- P depends on the drug concentration only if the brane sac are determined. This method allows drug molecules have a tendency to associate in evaluation of both passive and active transport. solution. For an ionizable drug, the following In the latter stages of preformulation testing equation is applicable: or early formulation studies, animals and humans (Conc. of drug in octanol) must be studied to assess the absorption effi - P = ciency and pharmacokinetic parameters and to [1−α ](Conc. of drug in water) establish possible in vitro and in vivo correlation for dissolution and bioavailability. where α equals the degree of ionization.

PHYSICAL PHARMACY CAPSULE 4.9 Partition Coeffi cient

The oil–water partition coeffi cient is a measure of a molecule’s lipophilic character; that is, its preference for the hydrophilic or lipophilic phase. If a solute is added to a mixture of two immiscible liquids, it will dis- tribute between the two phases and reach an equilibrium at a constant temperature. The distribution of the solute (unaggregated and undissociated) between the two immiscible layers can be described thus:

K = CU/CL where K is the distribution constant or partition constant,

CU is the concentration of the drug in the upper phase, and

CL is the concentration of the drug in the lower phase. This information can be effectively used in the 1. Extraction of crude drugs 2. Recovery of antibiotics from fermentation broth 3. Recovery of biotechnology-derived drugs from bacterial cultures 4. Extraction of drugs from biologic fl uids for therapeutic drug monitoring 5. Absorption of drugs from dosage forms (ointments, suppositories, patches) 6. Study of the distribution of fl avoring oil between oil and water phases of 7. In other applications



This basic relationship can be used to calculate the quantity of drug extracted from or remaining behind in a given layer and to calculate the number of extractions required to remove a drug from a mixture. The concentration of drug found in the upper layer (U) of two immiscible layers is given thus: U = Kr/(Kr + 1) where K is the distribution partition constant and

r is Vu/V1, or the ratio of the volume of upper and lower phases. The concentration of drug remaining in the lower layer (L) is given thus: L = 1/(Kr + 1)

If the lower phase is successively extracted again with n equal volumes of the upper layer, each upper (Un) contains the following fraction of the drug:

n Un = Kr/(Kr + 1) where

Un is the fraction contained in the nth extraction and n is the nth successive volume.

The fraction of solute remaining in the lower layer (Ln) is given thus:

n Ln = 1/(Kr + 1) More effi cient extractions are obtained using successive small volumes of the extraction solvent than single larger volumes. This can be calculated as follows when the same volume of extracting solvent is used in

divided portions. For example, the fraction Ln remaining after the nth extraction:

= 1 Ln ⎛⎞Kr n ⎜⎟+1 ⎝⎠n EXAMPLE 1 At 25°C and pH 6.8, the K for a second generation cephalosporin is 0.7 between equal volumes of butanol

and the fermentation broth. Calculate the U, L, and Ln (using the same volume divided into fourths). U = 0.7/(0.7 + 1) = 0.41, the fraction of drug extracted into the upper layer L = 1/(0.7 + 1) = 0.59, the fraction of drug remaining in the lower layer The total of the fractions in the U and L = 0.41 + 0.59 = 1. If the fermentation broth is extracted with four successive extractions accomplished by dividing the quantity of butanol used into fourths, the quantity of drug remaining after the fourth extraction is 1 L ==0.525 4 th ⎛⎞0.7× 1 4 ⎜⎟+ 1 ⎝⎠4 From this, the quantity remaining after a single volume, single extraction is 0.59, but when the single volume is divided into fourths and four successive extractions are done, the quantity remaining is 0.525; therefore, more was extracted using divided portions of the extracting solvent. Inherent in this procedure is the selection of appropriate extraction , drug stability, use of salting-out additives, and environ- mental concerns.


pKa/Dissociation Constants distribution, and elimination. The dissociation constant, or pKa, is usually determined by Among the physicochemical characteristics of potentiometric titration. For the practicing interest is the extent of dissociation or ioniza- pharmacist, it is important in predicting pre- tion of drug substances. This is important cipitation in admixtures and in calculating the because the extent of ionization has an impor- solubility of drugs at certain pH values. Physical tant effect on the formulation and pharmacoki- Pharmacy Capsule 4.10, pKa/Dissociation netic parameters of the drug. The extent of Constants, presents a brief summary of disso- dissociation or ionization in many cases is ciation and ionization concepts. highly dependent on the pH of the medium containing the drug. In formulation, often the DRUG AND DRUG PRODUCT vehicle is adjusted to a certain pH to obtain a STABILITY certain level of ionization of the drug for solu- bility and stability. In the pharmacokinetic One of the most important activities of prefor- area, the extent of ionization of a drug has a mulation work is evaluation of the physical and strong effect on its extent of absorption, chemical stability of the pure drug substance. It

PHYSICAL PHARMACY CAPSULE 4.10 pKa/Dissociation Constants

The dissociation of a weak acid in water is given by this expression:

HA « H++A– + – K1[HA] « K2[H ][A ]

At equilibrium, the reaction rate constants K1 and K2 are equal. This can be rearranged, and the dissocia- tion constant defi ned as

+ − ==K1 [H ][A ] Ka K2 [HA]

where Ka is the acid dissociation constant. For the dissociation of a weak base that does not contain a hydroxyl group, the following relationship can be used:

BH++↔+ H B The dissociation constant is described by [H+ ][B] K = a [BH+ ] The dissociation of a hydroxyl-containing weak base, +↔− + B H2 O OH +BH The dissociation constant is described by [OH−+ ][BH ] K = b [B] The hydrogen ion concentrations can be calculated for the solution of a weak acid using + = [H ] Ka C Similarly, the hydroxyl ion concentration for a solution of a weak base is approximated by − = [OH ] Kb C



Some practical applications of these equations are as follows.


−4 The Ka of lactic acid is 1.387 × 10 at 25°C. What is the hydrogen ion concentration of a 0.02 M solution? [H+− ]= 1.387 ×× 1043 0.02 = 1.665 × 10 − G- ion/L


−7 The Kb of morphine is 7.4 × 10 . What is the hydroxyl ion concentration of a 0.02 M solution? − [OH ]= 7.4 ×× 10−−74 0.02 = 1.216 × 10 G- ion/L

is essential that these initial studies be because a great number of medicinal agents are conducted using drug samples of known purity. esters or contain such other groupings as substi- The presence of impurities can lead to errone- tuted amides, lactones, and lactams, which are ous conclusions in such evaluations. Stability susceptible to the hydrolytic process (2). studies conducted in the preformulation phase Another destructive process is oxidation, include solid-state stability of the drug alone, which destroys many drug types, including alde- solution phase stability, and stability in the hydes, alcohols, phenols, , alkaloids, and presence of expected excipients. Initial investi- unsaturated fats and . Chemically, oxidation is gation begins with knowledge of the drug’s loss of electrons from an atom or a molecule. chemical structure, which allows the preformu- Each electron lost is accepted by some other lation scientist to anticipate the possible degra- atom or molecule, reducing the recipient. In dation reactions. inorganic , oxidation is accompanied by an increase in the positive valence of an element, Drug Stability: Mechanisms for example, ferrous (+2) oxidizing to ferric (+3). of Degradation In organic chemistry, oxidation is frequently con- sidered synonymous with the loss of hydrogen Chemical instability of medicinal agents may (dehydrogenation) from a molecule. Oxidation take many forms because the drugs in use today frequently involves free chemical radicals, which are of such diverse chemical constitution. Chem- are molecules or atoms containing one or more ically, drug substances are alcohols, phenols, unpaired electrons, such as molecular (atmo- aldehydes, ketones, esters, ethers, acids, salts, spheric) oxygen (•O—O•) and free hydroxyl alkaloids, glycosides, and others, each with (•OH). These radicals tend to take electrons from reactive chemical groups having different sus- other chemicals, thereby oxidizing the donor. ceptibilities to chemical instability. Chemically, Many of the oxidative changes in pharmaceu- the most frequently encountered destructive tical preparations have the character of autoxida- processes are hydrolysis and oxidation. tions. Autoxidations occur spontaneously under Hydrolysis is a solvolysis process in which the initial infl uence of atmospheric oxygen and (drug) molecules interact with water molecules proceed slowly at fi rst and then more rapidly. to yield breakdown products. For example, aspi- The process has been described as a type of rin, or acetylsalicylic acid, combines with a water chain reaction commencing with the union of molecule and hydrolyzes into one molecule of oxygen with the drug molecule and continuing salicylic acid and one molecule of acetic acid. with a free radical of this oxidized molecule Hydrolysis is probably the most important participating in the destruction of other drug single cause of drug decomposition, mainly molecules and so forth.


In drug product formulation work, steps are zero-order and fi rst-order reactions are taken to reduce or prevent deterioration due to encountered in pharmacy. These are presented hydrolysis, oxidation, and other processes. These in Physical Pharmacy Capsule 4.11, Rate Reac- techniques are discussed later. tions, along with some appropriate examples.

Drug and Drug Product Stability: Q Method of Shelf Life Estimation Kinetics and Shelf Life 10

The Q10 method of shelf life estimation lets the Stability is the extent to which a product retains pharmacist estimate shelf life for a product that within specifi ed limits and throughout its period has been stored or is going to be stored under a of storage and use (i.e., its shelf life) the same different set of conditions. It is explained in properties and characteristics that it possessed at Physical Pharmacy Capsule 4.12, Q10 Method of the time of its manufacture. Shelf Life Estimation. Five types of stability concern pharmacists:

1. Chemical: Each active ingredient retains its Enhancing Stability of Drug Products chemical integrity and labeled potency within the specifi ed limits. Many pharmaceutical ingredients may be used 2. Physical: The original physical properties, to prepare the desired dosage form of a drug including appearance, palatability, unifor- substance. Some of these agents may be used to mity, dissolution, and suspendability are achieve the desired physical and chemical char- retained. acteristics of the product or to enhance its 3. Microbiologic: Sterility or resistance to micro- appearance, odor, and taste. Other substances bial growth is retained according to the speci- may be used to increase the stability of the drug fi ed requirements. agents substance, particularly against hydrolysis and retain effectiveness within specifi ed limits. oxidation. In each instance, the added pharma- 4. Therapeutic: The therapeutic effect remains ceutical ingredient must be compatible with and unchanged. must not detract from the stability of the drug 5. Toxicologic: No signifi cant increase in substance. occurs. There are several approaches to the stabiliza- tion of pharmaceutical preparations containing Chemical stability is important for selecting stor- drugs subject to hydrolysis. Perhaps the most age conditions (temperature, light, humidity), obvious is the reduction or elimination of water selecting the proper container for dispensing (glass from the pharmaceutical system. Even solid dos- vs. plastic, clear vs. amber or opaque, cap liners), age forms containing water-labile drugs must be and anticipating interactions when mixing drugs protected from humidity in the atmosphere. and dosage forms. Stability and expiration dating This may be accomplished by applying a water- are based on reaction kinetics, that is, the study of proof protective coating over tablets or by keep- the rate of chemical change and the way this rate ing the drug in a tightly closed container. It is is infl uenced by concentration of reactants, prod- fairly common to detect hydrolyzed aspirin by ucts, and other chemical species and by factors noticing an odor of acetic acid upon opening a such as solvent, pressure, and temperature. bottle of aspirin tablets. In liquid preparations, In considering chemical stability of a pharma- water can frequently be replaced or reduced in ceutical, one must know the reaction order and the formulation through the use of substitute reaction rate. The reaction order may be the liquids such as glycerin, , and overall order (the sum of the exponents of the alcohol. In certain injectable products, anhy- concentration terms of the rate expression), or drous vegetable oils may be used as the drug’s the order with respect to each reactant (the solvent to reduce the chance of hydrolytic exponent of the individual concentration term in decomposition. the rate expression). Decomposition by hydrolysis may be prevented in other liquid drugs by suspen- Rate Reactions ding them in a nonaqueous vehicle rather than The reaction rate is a description of the drug con- dissolving them in an aqueous solvent. In still centration with respect to time. Most commonly, other instances, particularly for certain unstable



ZERO-ORDER RATE REACTIONS If the loss of drug is independent of the concentration of the reactants and constant with respect to time (i.e., 1 mg/mL/hour), the rate is called zero order. The mathematical expression is −dC = k dt 0

where k0 is the zero-order rate constant [concentration(C)/time(t)]. The integrated and more useful form of the equation: −+ C = k00 t C

where C0 is the initial concentration of the drug.

The units for a zero rate constant k0 are concentration per unit time, such as moles per liter-second or milligrams per milliliter per minute. It is meaningless to attempt to describe the time required for all material in a reaction to decompose,

that is, infi nity. Therefore, reaction rates are commonly described by k or by their half-life, t1/2. The half-life equation for a zero-order reaction: = t1/2 (1/2)(Co /k o )

If the Co changes, the t1/2 changes. There is an inverse relationship between the t½ and k. EXAMPLE 1 A drug suspension (125 mg/mL) decays by zero-order kinetics with a reaction rate constant of 0.5 mg/mL/

hour. What is the concentration of intact drug remaining after 3 days (72 hours), and what is its t1/2? C = −(0.5 mg/mL/hour) (72 hour) + 125 mg/mL C = 89 mg/mL after 3 days = t½ 1/2 (125 mg/mL)/(0.5 mg/mL/hour) = t½ 125 hours EXAMPLE 2 How long will it take for the suspension to reach 90% of its original concentration? 90%×= 125mg/mL 112.5mg/mL

C−− C 112.5mg/mL 125mg/mL t =−0 =25hours −− k0 0.5mg/mL/hour Drug suspensions are examples of pharmaceuticals that ordinarily follow zero-order kinetics for degradation.

FIRST-ORDER RATE REACTIONS If the loss of drug is directly proportional to the concentration remaining with respect to time, it is called a fi rst-order reaction and has the units of reciprocal time, that is, time−1. The mathematical expression is −dC = kC dt where C is the concentration of intact drug remaining, t is time, (dC/dt) is the rate at which the intact drug degrades, and k is the specifi c reaction rate constant.



The integrated and more useful form of the equation: −kt log C=+ logC 2.303 0

where C0 is the initial concentration of the drug. In natural log form, the equation is =− ln C kt + ln C0 The units of k for a fi rst-order reaction are per unit of time, such as per second. The half-life equation for a fi rst-order reaction is = t1/2 0.693/k

and can be easily derived from the fi rst-order equation by substituting values of C = 50% and Co = 100%, representing a decrease in concentration by 50%.

EXAMPLE 3 An ophthalmic solution of a mydriatic drug at 5 mg/mL exhibits fi rst-order degradation with a rate of 0.0005/day. How much drug will remain after 120 days, and what is its half-life? ln C = −(0.0005/day) (120) + ln (5 mg/mL) ln C = −0.06 + 1.609 ln C = 1.549 C = 4.71 mg/mL = t½ 0.693/0.0005/day = t½ 1,386 days

EXAMPLE 4 In Example 3, how long will it take for the drug to degrade to 90% of its original concentration? 90% of 5mg/mL= 4.5mg/mL ln4.5mg/mL=− (0.0005 / day)t+ ln(5mg/mL) ln4.5mg/mL− ln5mg/mL t = −0.0005 / day t= 210days


Stability projections for shelf life (t90, or the time required for 10% of the drug to degrade with 90% of the intact drug remaining) are commonly based on the Arrhenius equation: k Ea(T− T ) log ==2 21 k1 2.3RT12 T which relates the reaction rate constants (k) to temperatures (T) with the gas constant (R) and the energy of activation (Ea). The relationship of the reaction rate constants at two different temperatures provides the energy of activation for the degradation. By performing the reactions at elevated temperatures instead of allowing the process to proceed slowly at room temperature, the Ea can be calculated and a k value for room tem- perature determined with the Arrhenius equation.



EXAMPLE 5 The degradation of a new drug follows fi rst-order kinetics and has fi rst-order degradation rate constants of 0.0001 per hour at 60°C and 0.0009 at 80°C. What is its Ea? (0.0009) Ea (353− 333) log == (0.0001) (2.3)(1.987)(353)(333) Ea= 25,651kcal/mol


Q10 Method of Shelf Life Estimation

The Q10 approach, based on Ea, which is independent of reaction order, is described as = {(Ea/R)[(1/T + 10) - (1/T)]} Qe10 where Ea is the energy of activation, R is the gas constant, and T is the absolute temperature.

In usable terms, Q10, the ratio of two different reaction rate constants, is defi ned thus: K = (T+10) Q10 K T The commonly used Q values of 2, 3, and 4 relate to the energies of activations of the reactions for tem- peratures around room temperature (25°C). For example, a Q value of 2 corresponds to an Ea (kcal/mol) of 12.2, a Q value of 3 corresponds to an Ea of 19.4, and a Q value of 4 corresponds to an Ea of 24.5. Reasonable estimates can often be made using the value of 3.

The equation for Q10 shelf life estimates is

= t90 (T 1 ) t90 (T 2 ) (Δ T/10) Q10 where

t90T2 is the estimated shelf life,

t90T1 is the given shelf life at a given temperature, and Δ T is the difference in the temperatures T1 and T2. As is evident from this relationship, an increase in ΔT will decrease the shelf life and a decrease in ΔT will increase shelf life. This is the same as saying that storing at a warmer temperature will shorten the life of the drug and storing at a cooler temperature will increase the life of the drug.

EXAMPLE 1 An antibiotic solution has a shelf life of 48 hours in the refrigerator (5°C). What is its estimated shelf life at room temperature (25°C)? Using a Q value of 3, we set up the relationship as follows:

=t90 (T 1 ) = 48 == 48 t90 (T 2 ) (Δ− T/10) [(25 5)/10] 2 5.33hours Q310 3



EXAMPLE 2 An ophthalmic solution has a shelf life of 6 hours at room temperature (25°C). What is the estimated shelf life in a refrigerator at 5°C? (Note: Since the temperature is decreasing, ΔT will be negative.)

=66 = =×2 = t (T ) −−6 3 54hours 90 2 33[(5 25)/10] 2 These are estimates, and actual energies of activation can often be obtained from the literature for more exact calculations.

antibiotic drugs, when an aqueous preparation is providing electrons and easily available hydrogen desired, the drug may be supplied to the phar- atoms that are accepted more readily by the free macist in a dry form for reconstitution by adding radicals than are those of the drug being pro- a specifi ed volume of purifi ed water just before tected. Various antioxidants are employed in dispensing. The dry powder is actually a mixture pharmacy. Among those most frequently used in

of the antibiotic, suspending agents, fl avorants, aqueous preparations are sodium sulfi te (Na2SO3,

and colorants; when reconstituted by the phar- at high pH values), sodium bisulfi te (NaHSO3, at macist, it remains stable for the period over intermediate pH values), sodium metabisulfi te

which the preparation is normally consumed. (Na2S2O5 at low pH values), hypophosphorous

Refrigeration is advisable for most preparations acid (H3PO2), and ascorbic acid. In oleaginous considered subject to hydrolysis. Together with (oily or unctuous) preparations, alpha-tocopherol, temperature, pH is a major determinant of the butyl hydroxy anisole, and ascorbyl palmitate fi nd stability of a drug prone to hydrolytic decompo- application. sition. Hydrolysis of most drugs depends on the In June 1987, U.S. Food and Drug Adminis- relative concentrations of the hydroxyl and tration (FDA) labeling regulations went into hydronium ions, and a pH at which each drug is effect requiring a warning about possible aller- optimally stable can be easily determined. For gic-type reactions, including anaphylaxis, in the most hydrolyzable drugs, optimum stability is on package insert for prescription drugs to whose the acid side, somewhere between pH 5 and 6. fi nal dosage form sulfi tes have been added. Therefore, through judicious use of buffering Sulfi tes are used as in many agents, the stability of otherwise unstable com- injectable drugs, such as antibiotics and local pounds can be increased. Buffers are used to anesthetics. Some inhalants and ophthalmic maintain a certain pH, as described in Physical preparations also contain sulfi tes, but relatively Pharmacy Capsule 4.13, Buffer Capacity. few oral drugs contain these chemicals. The Pharmaceutically, oxidation of a susceptible purpose of the regulation is to protect the esti- drug substance is most likely to occur when it is mated 0.2% of the population who are subject not kept dry in the presence of oxygen or when it to allergic reactions to the chemicals. Many is exposed to light or combined with other chem- sulfi te-sensitive persons have asthma or other ical agents without proper regard to their infl u- allergic conditions. Previous to the regulations ence on oxidation. Oxidation of a chemical in a dealing with prescription medication, the FDA pharmaceutical preparation is usually accompa- issued regulations for the use of sulfi tes in food. nied by an alteration in the color of that prepara- Asthmatics and other patients who may be tion. It may also result in precipitation or a sulfi te sensitive should be reminded to read the change in odor. labels of packaged and medications to The oxidative process is diverted and the sta- check for the presence of these agents. Sulfi te bility of the drug is preserved by agents called agents covered by the regulations are potas- antioxidants, which react with one or more com- sium bisulfi te, potassium metabisulfi te, sodium pounds in the drug to prevent progress of the bisulfi te, sodium metabisulfi te, sodium sulfi te, chain reaction. In general, antioxidants act by and sulfur dioxide. The FDA permits the use of



pH, buffers, and buffer capacity are especially important in drug product formulation, since they affect the drug’s solubility, activity, absorption, and stability and the patient’s comfort. A buffer is a system, usually an , that can resist changes in pH upon addition of an acid or base. Buffers are composed of a weak acid and its conjugate base or a weak base and its conjugate acid. Buffers are prepared by one of these processes: 1. Mixing a weak acid and its conjugate base or a weak base and its conjugate acid 2. Mixing a weak acid and a strong base to form the conjugate base or a weak base and a strong acid to form the conjugate acid Using the Henderson-Hasselbach equation: = pH pKa + log (base/acid) Remember that the acid is the proton donor and the base is the proton acceptor.

EXAMPLE 1 A buffer is prepared by mixing 100 mL of 0.2 M phosphoric acid with 200 mL of 0.08 M sodium phosphate −3 monobasic. What is the pH of this buffer? (Ka of phosphoric acid = 7.5 × 10 ) Moles acid = (0.2 mol/1000 mL)(100 mL) = 0.02 mol; (0.02 mol)/(0.3 L) = 0.067 M Moles base = (0.08 mol/1000 mL)(200 mL) = 0.016 mol; (0.016 mol)/(0.3 L) = 0.053 M pKa = –log 7.5 × 10−3 = 2.125 pH = 2.125 + log (0.016 mol/0.02 mol) = 2.028

EXAMPLE 2 Determine the pH of the buffer prepared as shown: Sodium acetate 50 g Conc. HCl 10 mL Water q.s. 2 L Helpful numbers: pKa acetic acid = 4.76 m.w. sodium acetate = 82.08 m.w. acetic acid = 60.05 m.w. HCl = 36.45 Conc. HCl, 44% HCl w/v

NaAc + HCI ® NaCI + HAc + NaAc (0.609 mol) (0.121 mol) (0.121 mol) (0.121 mol) (0.488 mol) HCl: {(10 mL) [(44 g)/(100 mL)] (l mol)/(36.45 g)} = 0.121 mol NaAc: {(50 g)[(1 mol)/(82.08 g)] = 0.609 mol (0.609 mol) − (0.121 mol) = 0.488 mol pH = 4.76 + log (0.488 mol)/(0.121 mol) = 5.367 The ability of a buffer solution to resist changes in pH upon the addition of an acid or a base is called buffer capacity (β) and is defi ned thus: β = ΔB/ Δ pH where ΔB is molar concentration of acid or base added, ΔpH is change in pH due to addition of acid or base, and ΔpH can be determined experimentally or calculated using the Henderson-Hasselbach equation.



EXAMPLE 3 If 0.2 mole of HCl is added to a 0.015 M solution of ammonium hydroxide and the pH falls from 9.5 to 8.9, what is the buffer capacity? ΔpH = 9.5 − 8.9 = 0.6 ΔB = 0.2 mol/L = 0.2 M b = 0.2 M/0.6 = 0.33 M

EXAMPLE 4 If 0.002 mole of HCl is added to the buffer in Example 1, what is its buffer capacity? After adding 0.002 mole HCl: + = H3PO4: 0.02 mol 0.002 mol 0.022 mol − = NaH2PO4: 0.016 mol 0.002 mL 0.014 mol pH = 2.125 + log (0.014 mol/0.022 mol) = 1.929 ΔpH = 2.028 − 1.929 = 0.099 ΔAB = 0.002 mol/0.3 L = 0.0067 M β = 0.0067 M/0.099 = 0.067 M Another approach to calculating buffer capacity involves the use of Van Slyke’s equation: β = 2.3C {Ka[H+ ]/(Ka[H +2 ]) } where C is the sum of the molar concentrations of the acid and base, and [H+] = 10−pH.

EXAMPLE 5 What is the Van Slyke buffer capacity of the buffer prepared in Example 1? C = 0.0067 M + 0.0053 M = 0.12 M Ka = 7.5 + 10−3 [H+] = 10–2.028 = 9.38 ´ 10−3 M b = 2.3(0.12 M){[(7.5 × 10−3M)(9.38 × 10−3M)]/[(7.5 × 10−3M)/(9.38 × 10−3 M)2]} = 0.68 M

sulfi tes in prescription products, with the Because oxygen may adversely affect their proper labeling, because there are no generally stability, certain pharmaceuticals require an suitable substitutes for sulfi tes to maintain oxygen-free atmosphere during preparation potency in certain medications. Some but not and storage. Oxygen may be present in pharma- all epinephrine injections contain sulfi tes. ceutical liquids in the airspace within the con- The proper use of antioxidants permits tainer or may be dissolved in the liquid vehicle. their specifi c application only after appropri- To avoid these exposures, oxygen-sensitive ate biomedical and pharmaceutical studies. In drugs may be prepared in the dry state and certain instances, other pharmaceutical addi- packaged in sealed containers with the air tives can inactivate a given antioxidant. In replaced by an inert gas such as , as other cases, certain antioxidants can react may liquid preparations. This is common prac- chemically with the drugs they were intended tice in commercial production of and to stabilize without a noticeable change in the ampuls of easily oxidizable preparations appearance of the preparation. intended for parenteral use.



PREPARATION CATEGORY MONOGRAPH OR LABEL WARNING Epinephrine bitartrate Adrenergic Do not use inhalation, , nasal, or ophthalmic ophthalmic solution solution if it is brown or contains a precipitate Epinephrine inhalation solution Epinephrine injection Epinephrine nasal solution Epinephrine ophthalmic solution Isoproterenol sulfate inhalation, Adrenergic Do not use inhalation or injection if it is pink to solution (bronchodilator) brown or contains a precipitate Isoproterenol inhalation solution Nitroglycerin tablets Antianginal To prevent loss of potency, keep in original container or supplemental container specifi cally labeled suitable for nitroglycerin tablets Paraldehyde Subject to oxidation to form acetic acid

Trace metals originating in the drug, solvent, Statements in the United States Pharmacopeia container, or stopper are a constant source of dif- (USP), as with those in Table 4.2, warn of the fi culty in preparing stable solutions of oxidizable oxidative decomposition of drugs and prepara- drugs. The rate of formation of color in epineph- tions. In some instances, the specifi c agent to rine solutions, for instance, is greatly increased employ as a stabilizer is mentioned in the mono- by the presence of ferric, ferrous, cupric, and graph, and in others the term “suitable stabilizer” chromic ions. Great care must be taken to elimi- is used. An example in which a particular agent is nate these trace metals from labile preparations designated for use is in the monograph for potas- by thorough purifi cation of the source of the sium iodide oral solution, USP. Potassium iodide contaminant or by chemically complexing or in solution is prone to photocatalyzed oxidation binding the metal through the use of specialized and the release of free iodine, with a resultant agents that make it chemically unavailable for -to-brown discoloration of the solution. participation in the oxidative process. These The use of light-resistant containers is essential chelating agents are exemplifi ed by calcium diso- to its stability. As a further precaution against dium edetate and ethylenediaminetetraacetic decomposition if the solution is not to be used acid. within a short time, the USP recommends the Light can also act as a catalyst to oxidation addition of 0.5 mg of sodium thiosulfate for each reactions, transferring its energy (photons) to gram of potassium iodide. In the event free drug molecules, making the latter more reactive iodine is released during storage, the sodium through increased energy capability. As a pre- thiosulfate converts it to colorless and soluble caution against acceleration of oxidation, sensi- sodium iodide: tive preparations are packaged in light-resistant I + 2Na S O → 2NaI + Na S O or opaque containers. 2 2 2 3 2 4 6 Because most drug degradations proceed In summary, for easily oxidizable drugs, the for- more rapidly as temperature increases, it is also mulation pharmacist may stabilize the prepara- advisable to maintain oxidizable drugs in a cool tion by the selective exclusion from the system of place. Another factor that can affect the stability oxygen, oxidizing agents, trace metals, light, of an oxidizable drug in solution is the pH of the heat, and other chemical catalysts to the oxida- preparation. Each drug must be maintained in tion process. Antioxidants, chelating agents, and solution at the pH most favorable to its stability. buffering agents may be added to create and This varies from preparation to preparation and maintain a favorable pH. must be determined on an individual basis for In addition to oxidation and hydrolysis, the drug in question. destructive processes include polymerization,


chemical decarboxylation, and deamination. Harmonization guidelines and guidances However, these processes occur less frequently provide working recommendations to support and are peculiar to only small groups of chemi- the regulatory requirements. Among these are cal substances. Polymerization is a reaction the following (4): between two or more identical molecules that • “Stability Testing of New Drug Substances forms a new and generally larger molecule. and Products” Formaldehyde is an example of a drug capable • “Quality of Biotechnological Products: Stabil- of polymerization. In solution it may - ity Testing of Biotechnology/Biological Drug ize to paraformaldehyde (CH2O)n, a slowly sol- uble crystalline substance that may cloud Products” the solution. The formation of paraformalde- • “Photostability Testing of New Drug Sub- hyde is enhanced by cool storage, especially in stances and Products” solutions with high concentrations of formalde- • “Stability Testing of New Dosage Forms” hyde. The offi cial formaldehyde solution con- Drug and drug product stability testing during tains approximately 37% formaldehyde and every stage of development is critical to the qual- according to the USP, should be stored at tem- ity of the product. Drug stability is important peratures not below 15°C (59°F). If the solu- during preclinical testing and in clinical (human) tion becomes cloudy upon standing in a cool trials to obtain a true and accurate assessment place, it usually may be cleared by gentle warm- of the product being evaluated. For a marketed ing. Formaldehyde is prepared by the limited drug product, assurance of stability is vital to its oxidation of methanol (methyl alcohol), and the safety and effectiveness during the course of its USP permits a residual amount of this material shelf life and use. to remain in the fi nal product, since it can retard The FDA-required demonstration of drug sta- the formation of paraformaldehyde. Formalde- bility is necessarily different for each stage of drug hyde solution must be maintained in a tight development, such as for a 2-week preclinical container because oxidation of the formalde- study, an early Phase I study, a limited Phase II hyde yields formic acid. trial, a pivotal Phase III clinical study, or for a new

(O) (O) drug application. As a program CH OH → HCHO → HCOOH 3 progresses, so do the requisite data to demon- methanol formaldehyde formic acid strate and document the product’s stability pro- Other organic drug molecules may be degraded fi le. Before approval for marketing, a product’s through processes in which one or more of stability must be assessed with regard to its for- their active chemical groups are removed. mulation; the infl uence of its pharmaceutical These processes may involve various catalysts, ingredients; the infl uence of the container and including light and enzymes. Decarboxylation closure; the and processing condi- and deamination are examples of such pro- tions (e.g., heat); packaging components; condi- cesses; the former is decomposition of an tions of storage; anticipated conditions of shipping, organic acid (R•COOH) and release of carbon temperature, light, and humidity; and anticipated dioxide gas, and the latter is removal of the duration and conditions of pharmacy shelf life nitrogen- containing group from an organic and patient use. Holding intermediate product amine. For example, , a , deterio- components (such as drug granulations for tab- rates rapidly in acid solutions as a result of lets) for long periods before processing into fi n- extensive deamination (3). Thus, most prepara- ished pharmaceutical products can affect the tions of insulin are neutralized to reduce the stability of both the intermediate component and rate of decomposition. the fi nished product. Therefore, in-process stabil- ity testing, including retesting of intermediate components, is important. Stability Testing Product containers, closures, and other pack- FDA’s Current Good Manufacturing Practice aging features must be considered in stability regulations include sections on stability and sta- testing. For instance, tablets or capsules pack- bility testing of pharmaceutical components aged in glass or plastic bottles require different and fi nished pharmaceutical products. In addi- stability test protocols from those for blister packs tion, FDA and International Conference on or strip packaging. Drugs particularly subject to


hydrolysis or oxidative decomposition must be The fi ndings may be presented graphically, by evaluated accordingly. And sterile products must plotting the drug concentration as a function of meet sterility test standards to ensure protection time. From the experimental data, the reaction against microbial contamination. All preserva- rate may be determined and a rate constant and tives must be tested for effectiveness in the fi n- half-life calculated. ished product. The use of exaggerated conditions of tem- As noted elsewhere in this section, drug prod- perature, humidity, light, and others to test the ucts must meet stability standards for long-term stability of drug is termed acceler- storage at room temperature and relative humid- ated stability testing. Accelerated temperature ity. Products are also subjected to accelerated stability studies, for example, may be conducted stability studies as an indication of shelf life sta- for 6 months at 40°C with 75% relative humid- bility. It is an FDA requirement that if the data ity. If a signifi cant change in the product occurs are not submitted in the approved application, under these conditions, lesser temperature and the fi rst three postapproval production batches humidity may be used, such as 30°C and 60% of a drug substance must be subjected to long- relative humidity. Short-term accelerated stud- term stability studies and the fi rst three postap- ies are used to determine the most stable of the proval production batches of drug product must proposed formulations for a drug product. In be subjected to both long-term and accelerated stress testing, temperature elevations in 10°C stability studies (5, 6). increments higher than used in accelerated Drug instability in pharmaceutical formula- studies are employed until chemical or physical tions may be detected in some instances by a degradation. Once the most stable formulation change in the physical appearance, color, odor, is ascertained, its long-term stability is pre- taste, or texture of the formulation, whereas in dicted from the data generated from continu- other instances chemical changes may not be ing stability studies. Depending on the types self-evident and may be ascertained only through and severity of conditions employed, it is fairly chemical analysis. Scientifi c data pertaining to common to maintain samples under exagger- the stability of a formulation can lead to predic- ated conditions of both temperature and vary- tion of the expected shelf life of the proposed ing humidity for 6 to 12 months. Such studies product, and when necessary to redesign of the lead to the prediction of shelf life for a drug drug (e.g., into more stable salt or ester form) product. and to reformulation of the dosage form. Obvi- In addition to the accelerated stability stud- ously, the rate at which a drug product degrades ies, drug products are subjected to long-term is of prime importance. The study of the rate of stability studies under the usual conditions of chemical change and the way it is infl uenced by transport and storage expected during product such factors as the concentration of the drug or distribution. In conducting these studies, the reactant, the solvent, temperature and pressure, different national and international climate and other chemical agents in the formulation is zones to which the product may be subjected reaction kinetics. must be borne in mind and expected variances In general, a kinetic study begins by mea- in conditions of temperature and humidity suring the concentration of the drug at given included in the study design. Geographic regions intervals under a specifi c set of conditions are defi ned by zones: zone I, temperate; zone II, including temperature, pH, ionic strength, subtropical; zone III, hot and dry; and zone IV, light intensity, and drug concentration. The hot and humid. A given drug product may encoun- measurement of the drug’s concentration at the ter more than a single zone of temperature and various times reveals the stability or instability humidity variations during its production and of the drug under the specifi ed conditions with shelf life. Furthermore, it may be warehoused, the passage of time. From this starting point, transported, placed on a pharmacy’s shelf, and each of the original conditions may be varied to subsequently in the patient’s medicine cabinet, determine the infl uence of such changes on the over a varying time course and at a wide range drug’s stability. For example, the pH of the of temperature and humidity. In general, how- solution may be changed while the temperature, ever, the long-term (12 months minimum) test- light intensity, and original drug concentration ing of new drug entities is conducted at 25°C ± are held constant. 2°C and at a relative humidity of 60% ± 5%.


Samples maintained under these conditions may In addition, signs of degradation of the be retained for 5 years or longer, during which specifi c dosage forms must be observed and time they are observed for physical signs of reported. For the various dosage forms, this deterioration and chemically assayed. These includes the following (1): studies, considered with the accelerated stability Tablets: Appearance (cracking, chipping, mottling), fria- studies previously performed, lead to a more bility, hardness, color, odor, moisture content, clumping, precise determination of drug product stability, disintegration, and dissolution. actual shelf life, and the possible extension of Capsules: Moisture tackiness, color, appearance, shape, expiration dating. brittleness, and dissolution. When chemical degradation products are Oral solutions and suspensions: Appearance, precipita- detected, the FDA requires the manufacturer to tion, pH, color, odor, redispersibility (suspensions), and report their chemical identity, including struc- clarity (solutions). ture, mechanism of formation, physical and Oral powders: Appearance, color, odor, and moisture. chemical properties, procedures for isolation Metered-dose inhalation aerosols: Delivered dose per and purifi cation, specifi cations and directions for actuation, number of metered doses, color, particle size determination at levels expected to be present in distribution, loss of propellant, pressure, valve corrosion, the pharmaceutical product, and their pharma- spray pattern, and absence of pathogenic microorganisms. cologic action and biologic signifi cance, if any. Topical nonmetered aerosols: Appearance, odor, pres- Physical Pharmacy Capsule 4.14 Analytical sure, weight loss, net weight dispensed, delivery rate, and Methods and Standard Curves discusses some spray pattern. analytical methods and standard curve construc- Topical creams, ointments, , solutions, and : tion used in studies of this type. Appearance, color, homogeneity, odor, pH, resuspend-

PHYSICAL PHARMACY CAPSULE 4.14 Analytical Methods and Standard Curves

Any study involving concentration of a drug requires an analytical method and the development of stan- dard curves. There are numerous analytical methods used in pharmacy. It is important for pharmacists to have a basic understanding of pharmaceutical analysis to ensure that valid results are obtained when tests are being conducted. It is important to know (a) when to test, (b) what to test, (c) what method(s) to use, (d) how to interpret the results, (e) the limits of the test, and (f ) the importance of analytical testing in the overall quality program in pharmacy. The goal in analytical testing is to produce results as accurately, effi ciently, and quickly as possible. Any analytical method used should have accuracy, speed, reproducibility, and specifi city. No single analytical method is ideally suited for all drugs; each method has its own strengths and weaknesses, and there are a number of factors that determine the validity and reliability of results.

SELECTION OF AN ANALYTICAL METHOD One general consideration in analytical method selection is the type of information that is needed; quanti- tative (potency, concentration), semiquantitative (where a “cutoff” level is involved, as in endotoxin levels) or qualitative (yes/no type of testing, including substance identifi cation, sterility). Another consideration involves the physical and chemical characteristics of the analyte, including its solubility, partition coef- fi cient, dissociation constant (pKa), volatility, binding, and the quantity present. One must consider the degree of quantitative measurement in the validation process, for example accuracy, repeatability/reproducibility, and precision are required; generally, the greater the level that is required, the more sophisticated and expensive the analytical methods that must be used. This is also governed by the types of instrumentation that are on hand or available and the standards avail- able for comparison.



FACTORS INVOLVED IN METHODS SELECTION The ultimate analytical method selected depends upon a number of factors, including sample require- ments, sample handling/preparation/purifi cation requirements, type of data needed, and levels of speci- fi city and accuracy required.

SAMPLING REQUIREMENTS In any analytical method, there may be certain sample requirements that impact one’s choice, such as the number of samples needed, the diffi culty in obtaining a representative sample, the physical state of the sample (solid, liquid or gas), the type of container required for collection and storage of the sample (some analytes may sorb to the walls or cap liner of the sample containers), and leaching of the container material into the sample, if a liquid, may occur. All these may cause problems in analysis. In the event of sorption, siliconization of the sample vials may sometimes help. The storage requirements for the sample after collection must be specifi ed (type of container, material used, UV protection, contamination, etc.). The effects of air, such as oxidation of the sample ingre- dients, the presence of carbon dioxide and the formation of insoluble carbonates, pH changes, free versus bound drug, etc., must be considered. The sample must be stored at the proper temperature (refrigerated, frozen or controlled room) prior to shipment and during shipment. Procedures to follow if the sample is accidentally frozen or experiences a freeze-thaw cycle should be detailed. In considering the chemical and physical stability of the sample, the effects of water must also be considered. If the sample must be maintained in a dry environment, including a desiccant, this should be detailed. The stability of the sample during storage, extraction, and preparation must be determined. The potential for enzymatic breakdown, or other adverse effects of pH, temperature, solvents, bacterial growth, etc., must be addressed. If volatile solvents are required, special handling must be implemented to prevent evaporation because if some of the solvent is allowed to evaporate, the resulting concentration may be falsely elevated. The sample matrix effects must be determined. Any effects caused by sample (pipetting, aspi- ration), ionic strength (immunoassays, dialysis), buffers (ionized/unionized ratio can alter the extraction effi ciency of an analyte prior to analysis), and vapor pressure, where drug can be lost must be considered. If any sample pretreatment is required prior to shipment or working in-house, consider any inaccuracies that may occur from pipetting, which is one of the most common sources of analytical errors when work- ing with small volumes. There must be a consideration of any physical methods of separation and purifi cation that might be used. Most analytical methods require some degree of sample pretreatment to prepare it for analysis. These may include crystallization from solution, distillation, sublimation, solvent extraction, solid-phase extraction, chromatography, centrifugation; the proper choice of separation and purifi cation depends upon the physical and chemical properties of the sample, including its solubility, volatility, binding, quan- tity present, etc. The effect of any substances in the formulation that may interfere or alter the results must be known beforehand.

DATA INTERPRETATION REQUIREMENTS The collection of raw data from the analytical process must be done appropriately. One must ensure that appropriate and valid descriptive statistics are used to analyze the data, and that the operating param- eters of the analytical instruments are well established. Reference values, if available, should be provided with the analytical results. A description of the analytical controls used by the laboratory is important for documentation, as well as the source of reference standards used to establish standard curves.



ANALYTICAL METHODS In pharmaceutical analysis, analytical methods can be generally divided into physical testing methods, methods that interact with electromagnetic radiation, conductometric techniques, immunoassay meth- ods, separation techniques and others. Nonspecifi c methods generally include melting, freezing and boiling points, density, refractive index, polarimetry, ultraviolet/visible spectroscopy, and pH. Methods that are somewhat more specifi c include infrared spectroscopic, mass spectroscopy, ion selective electrodes, immunoassay methods, and chro- matographic methods (high performance liquid chromatography [HPLC] and gas chromatography [GC]), provided proper standards are used. Methods that can be routinely used for testing incoming bulk materials, whether active or excipients, include melting, freezing and boiling points, density, refractive index, UV/Visible spectroscopy, infrared spectroscopy, polarimetry, pH, and the separation methods. Final products may generally require a method such as HPLC or GC. A classifi cation of analytical methods follows along with suggested analyti- cal methods that can be used for different dosage forms.

CLASSIFICATION OF ANALYTICAL AND MICROBIOLOGICAL METHODS Physical testing procedures Melting point Freezing point Boiling point Density Refractive index Optical rotation (Polarimetry) Thermal analysis Color change Precipitate formation Viscosity change Interaction of electromagnetic radiation Ultraviolet/Visible spectroscopy Infrared spectroscopy Fluorescence/Phosphorescence spectroscopy Raman spectroscopy X-ray spectroscopy Flame emission and Atomic absorption spectroscopy Polarimetry Refractometry Interferometry Conductance methods pH Ion selective electrodes Polarography Immunoassay Radioimmunosassay Enzyme multiplied immunoassay technique Enzyme linked immunosorbent assay Fluorescent immunoassay



Separation techniques HPLC GC Thin layer chromatography Paper chromatography Column chromatography Gravimetric Balance Others Osmolality Microbiological methods Sterility testing Endotoxin testing effectiveness testing Suggested analytical methods for various dosage forms, depending upon the active drug:

DOSAGE ANALYTICAL METHOD Form Wt Vol pH Osm RI Sp Gr MP UV/vis HPLC GC IR Steril Endo Bulk substances — — * — * — * * * * * — — Powders * — — — — — — — * * — — — Capsules * — — — — — — — * * — — — Tablets *———————**—— Lozenges * — — — — — — — * * — — — Suppositories * — — — — * * — * * — — — Sticks *————**—**——— Solutions * * * * * * — * * * — — — Suspensions * * * — — * — — * * — — — Emulsions * * * — — * — — * * — — — Semisolids * — — — — * * — * * — — — Gels ***—**——**——— Ophthalmics, * * * * * * — * * * — *(Oph. — Otics & Nasals only) * * * * * — — * * * — * — Injections * * * * * * — * * * — * *

CONSTRUCTION OF A STANDARD CURVE A standard curve is constructed by analyzing samples (standards) of known composition, generally in increasing concentrations. As each standard is analyzed, an instrumental response (Absorbance, Peak Height, Peak Area, Other Numerical Value) will be obtained. The standard concentrations are plotted as the “x” axis on a graph and the instrumental responses are plotted on the “y” axis. As an example, The following table represents the results from an HPLC analytical method of methotrexate. Concentration (μg/mL) 0 10 20 30 Response (Peak Height in units) 0 2600 5190 7780



When plotted on a graph, one obtains the 10000 following: 8000 The next step involves analyzing the unknown sample to obtain a response from the instrument. 6000 For example, if the unknown sample provided 4000 Response an instrumental response of 3895, checking that 2000 value on the y-axis and moving toward the right 0 on the graph until it intersects the plotted line and 0 10 20 30 dropping down to the x-axis, we can read a value Conc (mg/mL) of 15 μg/mL of the methotrexate. As an option, the equation of the line can be calculated and the concentration determined by substituting the values of “y” and “b” with the slope of the line to obtain the drug concentration, as follows: m=Δ y/ Δ /x = (7780 − 0)/(30 − 0) = 7780/30 = 259.3 y = mx + b 3895 = 259.3 x + 0 x = 15.02 μg/mL

ability (lotions), consistency, particle-size distribution, Prescriptions requiring extemporaneous com- strength, and weight loss. pounding by the pharmacist do not require the Ophthalmic and nasal and oral inhalation preparations: extended shelf life that commercially manufac- Appearance, color, consistency, pH, clarity (solutions), tured and distributed products do because they particle size and resuspendability (suspensions, oint- are intended to be used immediately on receipt ments), strength, and sterility. by the patient and used only during the immedi- Small-volume parenterals: Appearance, color, particulate ate course of the prescribed treatment. How- matter, dispersibility (suspensions), pH, sterility, pyroge- ever, these compounded prescriptions must nicity, and closure integrity. remain stable and effi cacious during the course Large-volume parenterals: Appearance, color, clarity, of use, and the compounding pharmacist must particulate matter, pH, volume and extractables (when employ formulative components and techniques plastic containers are used), sterility, pyrogenicity, and closure integrity. that will result in a stable product (7). In years past, pharmacists were confronted pri- Suppositories: Softening range, appearance, and melting. marily with innocuous topical prescriptions that Emulsions: Appearance (such as ), color, required extemporaneous formulation. However, odor, pH, and viscosity. in recent years there has been a need to compound Controlled-release membrane drug delivery systems: Seal other drug delivery systems, for example, proges- strength of the drug reservoir, decomposition products, terone vaginal suppositories and oral suspensions, membrane integrity, drug strength, and drug release rate. from tablets or capsules. When presented with a Under usual circumstances, most manufactured prescription that requires extemporaneous com- products must have a shelf life of 2 or more pounding, the pharmacist is confronted with a dif- years to ensure stability at the time of consump- fi cult situation, because the potency and stability tion. Commercial products must bear an appro- of these prescriptions is a serious matter. Occa- priate expiration date that sets out the time sionally, the results of compatibility and stability during which the product may be expected to studies on such prescriptions are published in sci- maintain its potency and remain stable under entifi c and professional journals. These are very the designated storage conditions. The expira- useful; however, for some prescriptions stability tion date limits the time during which the prod- and compatibility information is not readily avail- uct may be dispensed by the pharmacist or used able. In these instances, it behooves the pharma- by the patient. cist to contact the manufacturer of the active


ingredient or ingredients to solicit stability infor- to prevent microbial growth, and stabilizers, mation. Also, a compilation of published stability such as antioxidants and chelating agents, may information is included in Trissel’s Stability of be used to prevent decomposition, as previously Compounded Formulations (8). The published discussed. In the preparation of tablets, diluents stability data are applicable only to products that or fi llers are commonly added to increase the are prepared identically to the products that are bulk of the formulation, binders to cause adhe- reported. sion of the powdered drug and pharmaceutical USP guidelines on stability of extemporaneous substances, antiadherents or lubricants to assist compounded formulations state that in the smooth tablet formation, disintegrating agents absence of stability information applicable to a to promote tablet breakup after administration, specifi c drug and preparation, the following and coatings to improve stability, control disin- guidelines can be used: nonaqueous liquids and tegration, or enhance appearance. Ointments, solid formulations in which the manufactured creams, and suppositories acquire their charac- drug is the source of the active ingredient, not teristic features from their pharmaceutical later than 25% of the time remaining until the bases. Thus, for each dosage form, the pharma- product’s expiration date or 6 months, whichever ceutical ingredients establish the primary fea- is earlier; nonaqueous liquids and solid formula- tures of the product and contribute to the phys- tions in which a USP or National Formulary (NF) ical form, texture, stability, taste, and overall substance is the source of active ingredient, a appearance. beyond-use date of 6 months; for water-containing Table 4.3 presents the principal categories of formulations prepared from ingredients in solid pharmaceutical ingredients, listing some of the form, a beyond-use date not later than 14 days in offi cial and commercial agents in use. Additional storage at cold temperatures; for all other formu- discussion of many ingredients may be found in lations, a beyond-use date of the intended dura- the chapters where they are most relevant; for tion of therapy or 30 days, whichever is earlier (7). example, pharmaceutical materials used in tab- Thus, if an oral aqueous liquid preparation is let and capsule formulations are discussed in made from a tablet or capsule formulation, the Chapters 7 and 8 and those used in modifi ed- pharmacist should make up only at most 14 days’ release solid oral dosage forms and drug delivery supply, and it must be stored in a refrigerator. systems in Chapter 9. Furthermore, the pharmacist must dispense the medication in a container conducive to stability HANDBOOK OF PHARMACEUTICAL and use and must advise the patient of the proper EXCIPIENTS AND FOOD AND method of use and conditions of storage of the CHEMICALS CODEX medication. Finally, when compounding on the basis of The Handbook of Pharmaceutical Excipients (9) extrapolated or less than concrete information, presents monographs on more than 250 excipi- the pharmacist is well advised to keep the for- ents used in dosage form preparation. Each mulation simple and not to shortcut but use the monograph includes such information as non- necessary pharmaceutical adjuvants to prepare proprietary, chemical, and commercial names; the prescription. empirical and chemical formulas and molecular weight; pharmaceutical specifi cations and chem- PHARMACEUTICAL INGREDIENTS ical and physical properties; incompatibilities and interactions with other excipients and drug AND EXCIPIENTS substances; regulatory status; and applications in DEFINITIONS AND TYPES pharmaceutical formulation or technology. Addi- tional excipients commonly used are listed in the To produce a drug substance in a fi nal dosage Food Chemicals Codex (FCC), now owned and form requires pharmaceutical ingredients. For published by the USP. The Codex contains infor- example, in the preparation of solutions, one or mation on general provisions and requirements more solvents are used to dissolve the drug sub- applying to specifi cations, tests and assays of the stance, fl avors and sweeteners are used to make FCC, monograph specifi cations, fl avor chemi- the product more palatable, colorants are added cals, infrared spectra, and general tests and to enhance appeal, preservatives may be added assays.



INGREDIENT TYPE DEFINITION EXAMPLES Acidifying agent Used in liquid preparations to provide acidic medium Citric acid for product stability Acetic acid Fumaric acid Hydrochloric acid Nitric acid Alkalinizing agent Used in liquid preparations to provide alkaline medium Ammonia solution for product stability Ammonium carbonate Diethanolamine Monoethanolamine Potassium hydroxide Sodium bicarbonate Sodium borate Sodium carbonate Sodium hydroxide Trolamine Adsorbent An agent capable of holding other molecules onto its Powdered surface by physical or chemical (chemisorption) means Activated charcoal Aerosol propellant Agent responsible for developing the pressure within Carbon dioxide an aerosol container and expelling the product when Dichlorodifl uoromethane the valve is opened Dichlorotetrafl uoroethane Trichloromonofl uoromethane Air displacement Agent employed to displace air in a hermetically sealed Nitrogen container to enhance product stability Carbon dioxide Antifungal preservative Used in liquid and semisolid preparations to prevent Butylparaben growth of fungi. Effectiveness of is usually Ethylparaben enhanced by use in combination Methylparaben Propylparaben Sodium benzoate Sodium propionate Antimicrobial preservative Used in liquid and semisolid preparations to prevent Benzalkonium chloride growth of microorganisms Antioxidant Used to prevent deterioration of preparations by Ascorbic acid oxidation Ascorbyl palmitate Butylated hydroxyanisole Butylated hydroxytoluene Hypophosphorous acid Monothioglycerol Propyl gallate Sodium ascorbate Sodium bisulfi te Sodium formaldehyde Sulfoxylate Sodium metabisulfi te Buffering agent Used to resist change in pH upon dilution or addition Potassium metaphosphate of acid or alkali Potassium phosphate, monobasic Sodium acetate Sodium citrate, anhydrous and dihydrate Chelating agent Substance that forms stable water-soluble Edetic acid complexes (chelates) with metals; used in some liquid Edetate disodium pharmaceuticals as stabilizers to complex heavy metals that might promote instability. In such use, they are also called sequestering agents (continued)



INGREDIENT TYPE DEFINITION EXAMPLES Colorant Used to impart color to liquid and solid (e.g., tablets FD&C Red No. 3 and capsules) preparations FD&C Red No. 20 FD&C Yellow No. 6 FD&C No. 2 D&C Green No. 5 D&C Orange No. 5 D&C Red No. 8 Caramel Ferric oxide, red Clarifying agent Used as a fi ltering aid for its adsorbent qualities Bentonite Emulsifying agent Used to promote and maintain dispersion of fi nely subdivided Acacia particles of liquid in a vehicle in which it is immiscible. End Cetomacrogol product may be a liquid or semisolid emulsion Cetyl alcohol (e.g., a ) Glyceryl monostearate Sorbitan monooleate Polyoxyethylene 50 stearate Encapsulating agent Used to form thin shells to enclose a drug for ease of adminis- Gelatin tration Flavorant Used to impart a pleasant fl avor and often odor to a oil preparation. In addition to the natural fl avorants listed, many Cinnamon oil synthetic ones are used Cocoa Menthol Orange oil Peppermint oil Vanillin Humectant Used to prevent drying of preparations, particularly ointments Glycerin and creams Propylene glycol Levigating agent Liquid used as an intervening agent to reduce the particle size Mineral oil of a powder by grinding, usually in a mortar Glycerin Propylene glycol Ointment base Semisolid vehicle for medicated ointments Lanolin Hydrophilic ointment Polyethylene glycol ointment Petrolatum Hydrophilic petrolatum White ointment Yellow ointment Rose water ointment Plasticizer Component of fi lm-coating solutions to make fi lm more pliable, Diethyl phthalate enhance spread of coat over tablets, beads, and granules Glycerin Solvent Used to dissolve another substance in preparation of a solution; Alcohol may be aqueous or not (e.g., oleaginous). Cosolvents, such as Corn oil water and alcohol (hydroalcoholic) and water and glycerin, may Cottonseed oil be used when needed. Sterile solvents are used in certain Glycerin preparations (e.g., injections) Isopropyl alcohol Mineral oil Peanut oil Purifi ed water Water for injection Sterile water for injection Sterile water for irrigation (continued)




Stiffening agent Used to increase thickness or hardness of a Cetyl alcohol preparation, usually an ointment Cetyl esters Microcrystalline wax Paraffi n Stearyl alcohol White wax Yellow wax base Vehicle for suppositories Cocoa Polyethylene glycols (mixtures) PEG 3350 (surface Substances that absorb to surfaces or interfaces to Benzalkonium chloride active agent) reduce surface or interfacial tension. May be used Nonoxynol 10 as wetting agents, , or emulsifying agents Octoxynol 9 Sodium lauryl sulfate Sorbitan monopalmitate Suspending agent Viscosity-increasing agent used to reduce sedimen- Agar tation rate of particles in a vehicle in which they are Bentonite not soluble; suspension may be formulated for oral, Carbomer (e.g., Carbopol) parenteral, ophthalmic, topical, or other route Carboxymethylcellulose sodium Hydroxyethyl cellulose Hydroxypropyl cellulose Hydroxypropyl methylcellulose Kaolin Methylcellulose Tragacanth Veegum Sweetening agent Used to impart sweetness to a preparation Dextrose Glycerin Mannitol Saccharin sodium Sorbitol Tablet antiadherents Prevent tablet ingredients from sticking to punches stearate and dies during production Tablet binders Substances used to cause adhesion of powder Acacia particles in tablet granulations Alginic acid Carboxymethylcellulose sodium Compressible (e.g., Nu-Tab) Ethylcellulose Gelatin Liquid glucose Methylcellulose Povidone Pregelatinized Tablet and capsule Inert fi ller to create desired bulk, fl ow properties, Dibasic calcium phosphate diluent and compression characteristics of tablets and Kaolin capsules Mannitol Microcrystalline cellulose Powdered cellulose Precipitated Sorbitol Starch (continued)



INGREDIENT TYPE DEFINITION EXAMPLES Tablet coating agent Used to coat a tablet to protect against decomposition by atmospheric oxygen or humidity, to provide a desired release pattern, to mask taste or odor, or for aesthetic purposes. Coating may be sugar, fi lm, or thick covering around a tablet. Sugar-coated tablets generally start to break up in the . Film forms a thin cover around a formed tablet or bead. Unless it is enteric, fi lm dissolves in the stomach. Enteric coating passes through the stomach to break up in the intestines. Some water-insoluble coatings (e.g., ethylcellulose) are used to slow the release of drug in the Sugar coating Liquid glucose Sucrose Film coating Hydroxyethyl cellulose Hydroxypropyl cellulose Hydroxypropyl methylcellulose Methylcellulose (e.g., Methocel) Ethylcellulose (e.g., Ethocel) Enteric coating Cellulose acetate phthalate (35% in alcohol, pharmaceutical glaze) Tablet direct compression Used in direct compression tablet formulations Dibasic calcium phosphate (e.g., Ditab) Tablet disintegrant Used in solid forms to promote disruption of the mass Alginic acid into smaller particles more readily dispersed or Polacrilin potassium dissolved (e.g., Amberlite) Sodium alginate Sodium starch glycolate Starch Tablet glidant Used in tablet and capsule formulations to improve fl ow Colloidal silica properties of the powder mixture Cornstarch Talc Tablet lubricant Used in tablet formulations to reduce friction during Calcium stearate tablet compression Magnesium stearate Mineral oil Stearic acid Zinc stearate Tablet or capsule opaquant Used to render a coating opaque. May be used alone or Titanium dioxide with a colorant Tablet polishing agent Used to impart an attractive sheen to coated tablets White wax Tonicity agent Used to render solution similar in osmotic-dextrose Sodium chloride characteristics to physiologic fl uids, e.g., in ophthalmic, parenteral, and irrigation fl uids Vehicle Carrying agent used in formulating a variety of liquids for oral and parenteral administration Generally, oral liquids are aqueous (e.g., syrups) or hydroalcoholic (e.g., ). Solutions for intravenous use are aqueous, whereas intramuscular injections may be aqueous or oleaginous (continued)




Flavored, sweetened Acacia syrup Aromatic syrup Aromatic Cherry syrup Cocoa syrup Orange syrup Syrup Oleaginous Corn oil Mineral oil Peanut oil Sesame oil Sterile Bacteriostatic sodium chloride injection Viscosity-increasing agent Used to render preparations more resistant to Alginic acid fl ow. Used in suspensions to deter sedimentation, Bentonite in ophthalmic solutions to enhance contact Carbomer time (e.g., methylcellulose), to thicken topical Carboxymethylcellulose creams, etc. Sodium Methylcellulose Povidone Sodium alginate Tragacanth

HARMONIZATION OF STANDARDS fl avors, colors, and preservatives, are discussed here. There is great interest in the international harmonization of standards applicable to phar- maceutical excipients. This is because the phar- APPEARANCE AND PALATABILITY maceutical industry is multinational, with major companies having facilities in more than a single Although most drug substances in use today country, with products sold in markets world- are unpalatable and unattractive in their natural wide, and with regulatory approval for these state, their preparations present them to the products required in each country. Standards for patient as colorful, fl avorful formulations attrac- each drug substance and excipient used in phar- tive to the sight, smell, and taste. These qualities, maceuticals are contained in pharmacopeias—or which are the rule rather than the exception, have for new agents, in an application for regulatory virtually eliminated the natural reluctance of many approval by the relevant governing authority. The patients to take medications because of disagree- four pharmacopeias with the largest international able odor or taste. In fact, the inherent attractive- use are the United States Pharmacopeia–National ness of today’s pharmaceuticals has caused them Formulary (USP–NF), British Pharmacopeia, to acquire the dubious distinction of being a European Pharmacopeia, and Japanese Pharma- source of accidental poisonings in the home, par- copeia. Uniform standards for excipients in these ticularly among children who are lured by their and other pharmacopeias would facilitate pro- organoleptic appeal. duction effi ciency, enable the marketing of a sin- There is some psychologic basis to drug gle formulation of a product internationally, and therapy, and the odor, taste, and color of a phar- enhance regulatory approval of pharmaceutical maceutical preparation can play a part. An products worldwide. The goal of harmonization appropriate drug has its most benefi cial effect is an ongoing effort by corporate representatives when it is accepted and taken properly by the and international regulatory authorities. patient. The proper combination of fl avor, fra- A few of the more common and widely used grance, and color in a pharmaceutical product pharmaceutical excipients, including sweeteners, contributes to its acceptance.


red and give it a banana taste and a mint odor. The color of a pharmaceutical must have a psy- chogenic balance with the taste, and the odor must also enhance that taste. Odor greatly affects the fl avor of a preparation or foodstuff. If one’s sense of smell is impaired, as during a head cold, the usual fl avor sensation of food is similarly diminished. The medicinal chemist and the formulation pharmacist are well acquainted with the taste characteristics of certain chemical types of drugs and strive to mask the unwanted taste through FIGURE 4.4 Electronic tongue to assist in formulation the appropriate use of fl avoring agents. Although development. (Courtesy of Alpha MOS.) there are no rules for unerringly predicting the taste sensation of a drug based on its chemical constitution, experience permits the presentation An “electronic tongue” is used to aid in pro- of several observations. For instance, although viding a global “taste fi ngerprint” during formu- we recognize and assume the salty taste of sodium lation development. It provides information on chloride, the formulation pharmacist knows that bitterness levels and the stability of fl avors in not all salts are salty but that their taste is a func- terms of taste (Figure 4.4). tion of both cation and anion. Whereas salty tastes are evoked by chlorides of sodium, potassium, Flavoring Pharmaceuticals and ammonium and by sodium bromide, bro- mides of potassium and ammonium elicit bitter The fl avoring of pharmaceuticals applies primar- and salty sensations, and potassium iodide and ily to liquids intended for oral administration. magnesium sulfate (epsom salt) are predomi- The 10,000 taste buds on the tongue, roof of the nantly bitter. In general, low–molecular-weight mouth, cheeks, and throat have 60 to 100 recep- salts are salty, and high-molecular-weight salts tor cells each (10). These receptor cells interact are bitter. with molecules dissolved in the saliva and pro- With organic compounds, an increase in the duce a positive or negative taste sensation. Med- number of hydroxyl groups (—OH) seems to ication in liquid form comes into immediate and increase the sweetness of the compound. direct contact with these taste buds. The addi- Sucrose, which has eight hydroxyl groups, is tion of fl avoring agents to liquid medication can sweeter than glycerin, another pharmaceutical mask the disagreeable taste. Drugs placed in sweetener, which has but three hydroxyl capsules or prepared as coated tablets may be groups. In general, the organic esters, alcohols, easily swallowed with no contact between the and aldehydes are pleasant to the taste, and drug and the taste buds. Tablets containing drugs since many of them are volatile, they also that are not especially distasteful may remain contribute to the odor and thus the fl avor of uncoated and unfl avored. Swallowing them with preparations in which they are used. Many water usually is suffi cient to avoid undesirable nitrogen-containing compounds, especially the taste sensations. However, chewable tablets, plant alkaloids (e.g., quinine) are extremely such as certain antacid and vitamin products, bitter, but certain other nitrogen-containing usually are sweetened and fl avored to improve compounds (e.g., aspartame) are extremely acceptance. sweet. The medicinal chemist recognizes that The fl avor sensation of a food or pharmaceu- even the most simple structural change in an tical is actually a complex blend of taste and organic compound can alter its taste. d-Glucose smell, with lesser infl uences of texture, tempera- is sweet, but l-glucose has a slightly salty taste; ture, and even sight. In fl avor-formulating a saccharin is very sweet, but N-methyl-saccha- pharmaceutical product, the pharmacist must rin is tasteless (11). give consideration to the color, odor, texture, and Thus, prediction of the taste characteristics of taste of the preparation. It would be incongru- a new drug is only speculative. However, it is ous, for example, to color a liquid pharmaceutical soon learned and the formulation pharmacist is


then put to the task of increasing the drug’s pal- Spice: Any aromatic vegetable substance in whole, bro- atability in the environment of other formulative ken, or ground form, except substances traditionally regarded as foods, such as onions, garlic, and celery; agents. The selection of an appropriate fl avoring whose signifi cant function in food is seasoning rather agent depends on several factors, primarily the than nutritional; that is true to name; and from which no taste of the drug substance itself. Certain fl avor- portion of any volatile oil or other fl avoring principle has ing materials are more effective than others in been removed. [CFR 101.22(a)(2)] masking or disguising the particular bitter, salty, In addition to the types of fl avors, you should sour, or otherwise undesirable taste of medicinal be aware of commercial fl avor designations, agents. Although individuals’ tastes and fl avor including the following (Note: ABCD would be preferences differ, cocoa-fl avored vehicles are the fl avor name, e.g., cherry): considered effective for masking the taste of bit- ter drugs. Fruit or citrus fl avors are frequently Natural ABCD fl avor All components derived from used to combat sour or acid-tasting drugs, and ABCD. cinnamon, orange, raspberry, and other fl avors ABCD fl avor, natural At least one component and have been successfully used to make prepara- artifi cial-derived from ABCD. tions of salty drugs more palatable. No defi nition of natural to The age of the intended patient should also artifi cial ratio. be considered in the selection of the fl avoring ABCD fl avor, WONFa All components natural. agent, because certain age groups seem to prefer At least one component derived from ABCD. certain fl avors. Children prefer sweet candy-like preparations with fruity fl avors, but adults seem Natural fl avor, ABCD type All components natural. No components derived to prefer less sweet preparations with a tart from ABCD. rather than a fruit fl avor. ABCD fl avor, artifi cial All components are artifi cial. can consist of oil- or water-soluble liq- uids and dry powders; most are diluted in carri- Conceptual fl avors May contain artifi cial fl avors. No reference point. ers. Oil-soluble carriers include soybean and May only have to declare in other edible oils; water-soluble carriers include ingredient declaration. water, ethanol, propylene glycol, glycerin, and a emulsifi ers. Dry carriers include maltodextrins, WONF, with other natural fl avors. corn syrup solids, modifi ed , , A general guide to using fl avors is to start as fol- salt, sugars, and whey protein. Flavors can lows (keep in mind it is usually possible to add degrade as a result of exposure to light, tempera- more fl avor, but once it is added, it is too late to ture, head space oxygen, water, enzymes, con- remove it). taminants, and other product components, so they must be carefully selected and checked for Water-soluble fl avors Generally start at 0.2% for artifi cial stability. and 1%–2% for natural fl avors. The different types of fl avors include natural, Oil-soluble fl avors Generally start at 0.1% in fi nished artifi cial, and spice: product for artifi cial fl avors and 0.2% for natural fl avors. Natural fl avor: Essential oil, oleoresin, essence or extrac- Powdered fl avors Generally start at 0.1% in fi nished tive, protein hydrolysate, distillate, or any product of product for artifi cial fl avors and roasting, heating, or enzymolysis, which contains the fl a- 0.75% for natural fl avors. voring constituents derived from a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, , sea- food, poultry, eggs, dairy products, or fermentation prod- Sweetening Pharmaceuticals ucts thereof whose signifi cant function in food is fl avor- In addition to sucrose, a number of artifi cial ing rather than nutritional. [CFR 101.22(a)(3)] In “all natural” fl avors, one doesn’t necessarily know the exact sweetening agents have been used in foods and chemical composition. pharmaceuticals over the years. Some of these, Artifi cial fl avor: Any substance used to impart fl avor that including aspartame, saccharin, and cyclamate, is not derived from a spice, fruit or fruit juice, vegetable have faced challenges over their safety by the or vegetable juice, edible yeast, herb, bark, bud, root, leaf FDA and restrictions to their use and sale; in or similar plant material, meat, fi sh, poultry, eggs, dairy products, or fermentation products thereof. [CFR fact, in 1969, FDA banned cyclamates from use 101.22(a)(1)] in the United States.


The introduction of diet soft drinks in the Academy of Sciences issued an interim report on 1950s provided the spark for the widespread use the safety of nonnutritive sweeteners, including of artifi cial sweeteners today. Besides dieters, saccharin. In the early 1970s, FDA began a major patients with diabetes are regular users of artifi - review of hundreds of food additives on the GRAS cial sweeteners. Over the years, each of the arti- list to determine whether current studies justifi ed fi cial sweeteners has undergone long periods of their safe status. In 1972, with new studies under review and debate. Critical to the evaluation of way, FDA decided to take saccharin off the GRAS food additives are issues of metabolism and tox- and establish interim limits that would permit its icity. For example, almost none of the saccharin continued use until additional studies were com- a person consumes is metabolized; it is excreted pleted. (Previous studies indicated that male and by the kidneys virtually unchanged. Cyclamate, female rats fed doses of saccharin developed a sig- on the other hand, is metabolized, or processed, nifi cant incidence of bladder tumors.) In Novem- in the digestive tract, and its by-products are ber 1977, Congress passed the Saccharin Study excreted by the kidneys. Aspartame breaks down and Labeling Act, which permitted saccharin’s in the body into three basic components: the continued availability while mandating that warn- amino acids phenylalanine and aspartic acid, and ing labels be used to advise consumers that sac- methanol. These three components, which also charin caused cancer in animals. The law also occur naturally in various foods, are in turn directed FDA to arrange further studies of car- metabolized through regular pathways in the cinogens and toxic substances in foods. body. Because of its metabolism to phenylala- Cyclamate was introduced into beverages nine, the use of aspartame by persons with phe- and foods in the 1950s and dominated the arti- nylketonuria (PKU) is discouraged, and diet fi cial sweetener market in the 1960s. After foods and drinks must bear an appropriate label much controversy regarding its safety, the FDA warning indicating that the particular foodstuff issued a fi nal ruling in 1980 stating that safety not be consumed by such individuals. They can- has not been demonstrated. Since that date, sci- not metabolize phenylalanine adequately, so entifi c studies have continued the search for they undergo an increase in the serum levels of conclusive support or rejection of the FDA the (hyperphenylalaninemia). This decision. At question is cyclamate’s possible can result in mental retardation and can affect carcinogenicity and its possible causation of the fetus of a pregnant woman who has PKU. genetic damage and testicular atrophy. See the Passage in 1958 of the Food Additives indicated references for a review of the recent Amendment to the Food, Drug, and Cosmetic history of sweeteners, including saccharin, Act produced a major change in how the federal cyclamate, fructose, polyalcohols, sucrose, and government regulates food additives. For one aspartame (12–15). thing, no new may be used if ani- Acesulfame potassium, a nonnutritive sweet- mal feeding studies or other appropriate tests ener discovered in 1967, was approved in 1992 by showed that it caused cancer. This is the famous the FDA. It previously was used in a number of Delaney Clause. The amount of the substance other countries. Structurally similar to saccharin, it one would have to consume to induce cancer is is 130 times as sweet as sucrose and is excreted not signifi cant under the Delaney Clause. unchanged in the urine. Acesulfame is more stable Another critical feature of the 1958 amend- than aspartame at elevated temperatures and FDA ment was that it did not apply to additives that initially approved it for use in candy, , were generally recognized by experts as safe for confectionery, and instant coffee and tea. their intended uses. Saccharin, cyclamate, and a A relatively new sweetening agent in U.S. long list of other substances were being used in commerce is Stevia powder, the extract from the foods before the amendment’s passage and were leaves of the plant Stevia rebaudiana bertoni. It “generally recognized as safe”—or what is known is natural, nontoxic, safe, and about 30 times as today as GRAS. Aspartame, on the other hand, sweet as cane sugar, or sucrose. It can be used in was the fi rst artifi cial sweetener to fall under the both hot and cold preparations. Table 4.4 com- 1958 amendment’s requirement for premarketing pares three of the most commonly used sweet- proof of safety, because the fi rst petition to FDA eners in the food and drug industry. for its approval was fi led in 1973. In 1968, the Most large pharmaceutical manufacturers Committee on Food Protection of the National have special laboratories for taste-testing



SUCROSE SACCHARIN ASPARTAME Source Sugar cane; Chemical synthesis; phthalic Chemical synthesis; methyl ester dipeptide sugar beet anhydride, a petroleum product of phenylalanine and aspartic acid Relative 1 300 180–200 sweetness Bitterness None Moderate to strong None Aftertaste None Moderate to strong; None sometimes metallic or bitter Calories 4/g 0 4/g Acid stability Good Excellent Fair Heat stability Good Excellent Poor

proposed formulations of their products. Panels The synthetic coloring agents used in of employees or interested community partici- pharmaceutical products were fi rst prepared in pants participate in evaluating the various for- the middle of the 19th century from principles mulations, and their assessments become the of coal tar. Coal tar (pix carbonis), a thick, black basis for the fi rm’s fl avoring decisions. viscid liquid, is a by-product of the destructive The fl avoring agent in liquid pharmaceutical distillation of coal. Its composition is extremely products is added to the solvent or vehicle com- complex, and many of its constituents may be ponent of the formulation in which it is most separated by fractional distillation. Among its soluble or miscible. That is, water-soluble fl a- products are anthracene, benzene, naphtha, cre- vorants are added to the aqueous component of osote, phenol, and pitch. About 90% of the dyes a formulation and poorly water-soluble fl a- used in the products FDA regulates are synthe- vorants are added to the alcoholic or other non- sized from a single colorless derivative of ben- aqueous solvent component of the formulation. zene called aniline. These aniline dyes are also In a hydroalcoholic or other multisolvent sys- known as synthetic organic dyes or as coal tar tem, care must be exercised to maintain the fl a- dyes, since aniline was originally obtained from vorant in solution. This is accomplished by bituminous coal. Aniline dyes today come mainly maintaining a suffi cient level of the fl avorant’s from petroleum. solvent. Many coal tar dyes were originally used indis- criminately in foods and beverages to enhance Coloring Pharmaceuticals their appeal without regard to their toxic poten- tial. It was only after careful scrutiny that some Coloring agents are used in pharmaceutical dyes were found to be hazardous to health preparations for esthetics. A distinction should because of either their own chemical nature or be made between agents that have inherent the impurities they carried. As more dyestuffs color and those that are employed as colorants. became available, some expert guidance and reg- Certain agents—sulfur (yellow), ribofl avin (yel- ulation were needed to ensure the safety of the low), cupric sulfate (blue), ferrous sulfate (bluish public. After passage of the Food and Drug Act green), cyanocobalamin (red), and red mercuric in 1906, the U.S. Department of Agriculture iodide (vivid red)—have inherent color and are established regulations by which a few colorants not thought of as pharmaceutical colorants in were permitted or certifi ed for use in certain the usual sense of the term. products. Today, the FDA regulates the use of Although most pharmaceutical colorants in color additives in foods, drugs, and use today are synthetic, a few are obtained from through the provisions of the Federal Food, natural mineral and plant sources. For example, Drug, and Cosmetic Act of 1938, as amended in red ferric oxide is mixed in small proportions 1960 with the Color Additive Amendments. Lists with zinc oxide powder to give calamine its char- of color additives exempt from certifi cation and acteristic pink color, which is intended to match those subject to certifi cation are codifi ed into law the skin tone upon application. and regulated by the FDA (16). Certifi ed color


additives are classifi ed according to their approved As a part of the National Toxicology Program use: (a) FD&C color additives, which may be of the U.S. Department of Health and Human used in foods, drugs, and cosmetics; (b) D&C Services, various substances, including color addi- color additives, some of which are approved for tives, are studied for toxicity and carcinogenesis. use in drugs, some in cosmetics, and some For color additives, the study protocols usually in medical devices; and (c) external D&C color call for a 2-year study in which groups of male additives, the use of which is restricted to exter- and female mice and rats are fed diets containing nal parts of the body, not including the lips or any various quantities of the colorant. The killed and other body surface covered by mucous membrane. surviving animals are examined for evidence of Each certifi cation category has a variety of basic long-term toxicity and carcinogenesis. Five cate- colors and shades for coloring pharmaceuticals. gories of evidence of carcinogenic activity are One may select from a variety of FD&C, D&C, used in reporting observations: (a) “clear evi- and external D&C reds, , oranges, greens, dence” of carcinogenic activity; (b) “some , and violets. By selective combinations of evidence”; (c) “equivocal evidence,” indicating the colorants one can create distinctive colors uncertainty; (d) “no evidence,” indicating no (Table 4.5). observable effect; and (e) “inadequate study,” for studies that cannot be evaluated because of major TABLE 4.5 EXAMPLES OF COLOR fl aws. FORMULATIONS The certifi cation status of the colorants is con- tinually reviewed, and changes are made in the list SHADE OR COLOR FD&C DYE % OF BLEND of certifi ed colors in accordance with toxicology Orange Yellow No. 6 100 fi ndings. These changes may be (a) the withdrawal or of certifi cation, (b) the transfer of a colorant from Yellow No. 5 95 one certifi cation category to another, or (c) the Red No. 40 5 addition of new colors to the list. Before gaining Cherry Red No. 40 100 certifi cation, a color additive must be demon- or Red No. 40 99 strated to be safe. In the case of pharmaceutical Blue No. 1 1 preparations, color additives, as with all additives, Strawberry Red No. 40 100 must not interfere with therapeutic effi cacy, nor or may they interfere with the prescribed assay pro- Red No. 40 95 cedure for the preparation. Red No. 3 5 In the 1970s, concern and scientifi c ques- Lemon Yellow No. 5 100 tioning of the safety of some color additives Lime Yellow No. 5 95 heightened. A color that drew particular atten- Blue No. 1 5 tion was FD&C Red No. 2, because of its exten- Grape Red No. 40 80 sive use in foods, drugs, and cosmetics. Blue No. 1 20 Researchers in Russia reported that this color, Raspberry Red No. 3 75 also known as amaranth, caused cancer in rats. Yellow No. 6 20 Although the FDA was never able to determine Blue No. 1 5 the purity of the amaranth tested in Russia, Butterscotch Yellow No. 5 74 these reports led to FDA investigations and a Red No. 40 24 series of tests that eventually resulted in with- Blue No. 1 2 drawal of FD&C Red No. 2 from the FDA Red No. 40 52 certifi ed list in 1976 because its sponsors were Yellow No. 5 40 Blue No. 1 8 unable to prove safety. That year, FDA also ter- minated approval for use of FD&C Red No. 4 Caramel Yellow No. 5 64 Red No. 3 21 in maraschino cherries and ingested drugs Yellow No. 6 9 because of unresolved safety questions. FD&C Blue No. 1 6 Red No. 4 is now permitted only in externally Cinnamon Yellow No. 5 60 applied drugs and cosmetics. Red No. 40 35 FD&C Yellow No. 5 (also known as tartra- Blue No. 1 5 zine) causes allergic-type reactions in many peo- From literature of Warner-Jenkinson Co., St. Louis, Mo. ple. People who are allergic to aspirin are also


likely to be allergic to this dye. As a result, the lets, a generally larger proportion of dye is required FDA requires listing of this dye by name on the (about 0.1%) to achieve the desired hue than with labels of foods (e.g., butter, cheese, ) liquid preparations. and ingested drugs containing it. Both dyes and lakes are used to color sugar- A colorant becomes an integral part of a phar- coated tablets, fi lm-coated tablets, direct- maceutical formulation, and its exact quantita- c ompression tablets, pharmaceutical suspen- tive amount must be reproducible each time the sions, and other dosage forms (17). Traditionally, formulation is prepared, or else the preparation sugar-coated tablets have been colored with would have a different appearance from batch to syrup solutions containing varying amounts of batch. This requires a high degree of skill, for the water-soluble dyes, starting with very dilute the amount of colorant generally added to liquid solutions, working up to concentrated color syrup preparations ranges from 0.0005% to 0.001% solutions. As many as 30 to 60 coats are common. depending upon the colorant and the depth of With the lakes, fewer color coats are used. color desired. Because of their color potency, Appealing tablets have been made with as few as dyes generally are added to pharmaceutical 8 to 12 coats using lakes dispersed in syrup. preparations in the form of diluted solutions Water-soluble dyes in aqueous vehicles or lakes rather than as concentrated dry powders. This dispersed in organic solvents may be effectively permits greater accuracy in measurement and sprayed on tablets to produce attractive fi lm more consistent color production. coatings. There is continued interest today in In addition to liquid dyes in the coloring of chewable tablets, because of the availability of pharmaceuticals, lake pigments may also be many direct-compression materials such as dex- used. Whereas a chemical material exhibits trose, sucrose, mannitol, sorbitol, and spray- coloring power or tinctorial strength when dis- dried lactose. The direct-compression colored solved, pigment is an insoluble material that chewable tablets may be prepared with 1 lb of colors by dispersion. An FD&C lake is a pig- lake per 1,000 lb of tablet mix. For aqueous sus- ment consisting of a substratum of alumina pensions, FD&C water-soluble colors or lakes hydrate on which the dye is adsorbed or pre- may be satisfactory. In other suspensions, FD&C cipitated. Having aluminum hydroxide as the lakes are necessary. The lakes, added to either substrate, the lakes are insoluble in nearly all the aqueous or nonaqueous phase, generally at a solvents. FD&C lakes are subject to certifi ca- level of 1 lb of color per 1,000 lb of suspension, tion and must be made from certifi ed dyes. require homogenization or mechanical blending Lakes do not have a specifi ed dye content; they to achieve uniform coloring. range from 10% to 40% pure dye. By their For the most part, ointments, suppositories, nature, lakes are suitable for coloring products and ophthalmic and parenteral products assume in which the moisture levels are low. the color of their ingredients and do not contain Lakes in pharmaceuticals are commonly used color additives. Should a dye lose the certifi ca- in the form of fi ne dispersions or suspensions. tion status it held when a product was fi rst for- The pigment particles may range in size from mulated, manufactured, and marketed, the less than 1 μm up to 30 μm. The fi ner the parti- manufacturer must reformulate within a reason- cle, the less chance for color speckling in the fi n- able length of time, using only color additives ished product. Blends of various lake pigments certifi ed at the new date of manufacture. may be used to achieve a variety of colors, and In addition to esthetics and the certifi cation various vehicles, such as glycerin, propylene status of a dye, a formulation pharmacist must glycol, and sucrose-based syrup, may be select the dyes to be used in a particular formula employed to disperse the colorants. on the basis of their physical and chemical prop- Colored empty gelatin capsule shells may be erties. Of prime importance is the solubility of a used to hold a powdered drug mixture. Many prospective dye in the vehicle to be used for a commercial capsules are prepared with a capsule liquid formulation or in a solvent to be employed body of one color and a cap of a different color, during a pharmaceutical process, as when the resulting in a two-colored capsule. This makes cer- dye is sprayed on a batch of tablets. In general, tain commercial products more readily identifi - most dyes are broadly grouped into those that able than solid-colored capsules. For powdered are water soluble and those that are oil soluble; drugs dispensed as such or compressed into tab- few if any dyes are both. Usually, a water-soluble


dye is also adequately soluble in commonly used also require an antimicrobial preservative to pharmaceutical liquids like glycerin, alcohol, and maintain their aseptic condition throughout stor- glycol ethers. Oil-soluble dyes may also be solu- age and use. Other types of preparations that are ble to some extent in these solvents and in liquid not sterilized during their preparation but are petrolatum (mineral oil), fatty acids, fi xed oils, particularly susceptible to microbial growth and . No great deal of solubility is required, because of the nature of their ingredients are since the concentration of dye in a given prepa- protected by the addition of an antimicrobial ration is minimal. preservative. Preparations that provide excellent Another important consideration when select- growth media for microbes are most aqueous ing a dye for use in a liquid pharmaceutical is the preparations, especially syrups, emulsions, sus- pH and pH stability of the preparation to be pensions, and some semisolid preparations, par- colored. Dyes can change color with a change in ticularly creams. Certain hydroalcoholic and pH, and the dye must be selected so that no most alcoholic preparations may not require the anticipated pH change will alter the color during addition of a chemical preservative when the the usual shelf life. The dye also must be chemi- alcoholic content is suffi cient to prevent micro- cally stable in the presence of the other formula- bial growth. Generally, 15% V/V alcohol will pre- tive ingredients and must not interfere with the vent microbial growth in acid media and 18% stability of the other agents. To maintain their V/V in alkaline media. Most alcohol-containing original colors, FD&C dyes must be protected pharmaceuticals, such as elixirs, spirits, and tinc- from oxidizing agents, reducing agents (espe- tures, are self-sterilizing and do not require cially metals, including iron, aluminum, zinc, additional preservation. The same applies to and tin), strong acids and alkalis, and excessive other individual pharmaceuticals that by virtue heating. Dyes must also be reasonably photo- of their vehicle or other formulative agents may stable; that is, they must not change color when not permit the growth of microorganisms. exposed to light of anticipated intensities and wavelengths under the usual conditions of shelf Preservative Selection storage. Certain medicinal agents, particularly those prepared in liquid form, must be protected When experience or shelf storage experiments from light to maintain their chemical stability indicate that a preservative is required in a and their therapeutic effectiveness. These prep- pharmaceutical preparation, its selection is based arations are generally kept in dark amber or on many considerations, including some of the opaque containers. For solid dosage forms of following: photolabile drugs, a colored or opaque capsule • The preservative prevents the growth of the shell may enhance the drug’s stability by shield- type of microorganisms considered the most ing out light rays. likely contaminants of the preparation. • The preservative is soluble enough in water to PRESERVATIVES achieve adequate concentrations in the aque- ous phase of a system with two or more In addition to the stabilization of pharmaceutical phases. preparations against chemical and physical deg- • The proportion of preservative remaining radation due to changed environmental condi- undissociated at the pH of the preparation tions within a formulation, certain liquid and makes it capable of penetrating the microor- semisolid preparations must be preserved against ganism and destroying its integrity. microbial contamination. • The required concentration of the preser- vative does not affect the safety or comfort Sterilization and Preservation of the patient when the pharmaceutical preparation is administered by the usual or Although some types of pharmaceutical prod- intended route; that is, it is nonirritating, ucts, for example, ophthalmic and injectable nonsensitizing, and nontoxic. preparations, are sterilized by physical methods • The preservative has adequate stability and (autoclaving for 20 minutes at 15 lb pressure and will not be reduced in concentration by chem- 121°C, dry heat at 180°C for 1 hour, or bacterial ical decomposition or volatilization during the fi ltration) during manufacture, many of them desired shelf life of the preparation.


• The preservative is completely compatible them unavailable for their preservative function. with all other formulative ingredients and It is essential for the research pharmacist to does not interfere with them, nor do they examine all formulative ingredients as one affects interfere with the effectiveness of the preser- the other to ensure that each agent is free to do vative agent. its job. In addition, the preservative must not • The preservative does not adversely affect the interact with a container, such as a metal oint- preparation’s container or closure. ment tube or a plastic medication bottle, or with an enclosure, such as a rubber or plastic cap or liner. Such an interaction could result in decom- General Preservative Considerations position of the preservative or the container clo- Microorganisms include molds, yeasts, and bac- sure or both, causing decomposition and teria, with bacteria generally favoring a slightly contamination. Appropriate tests should be alkaline medium and the others an acid medium. devised and conducted to prevent this type of Although few microorganisms can grow below preservative interaction. pH 3 or above pH 9, most aqueous pharmaceuti- cal preparations are within the favorable pH range and therefore must be protected against Preservatives interfere with microbial growth, microbial growth. To be effective, a preservative multiplication, and metabolism through one or agent must be dissolved in suffi cient concentra- more of the following mechanisms: tion in the aqueous phase of a preparation. Fur- thermore, only the undissociated fraction or • Modifi cation of permeability molecular form of a preservative possesses pre- and leakage of cell constituents (partial lysis) servative capability, because the ionized portion • Lysis and cytoplasmic leakage is incapable of penetrating the . • Irreversible coagulation of cytoplasmic con- Thus, the preservative selected must be largely stituents (e.g., protein precipitation) undissociated at the pH of the formulation being • Inhibition of cellular metabolism, such as by prepared. Acidic preservatives like benzoic, boric, interfering with enzyme systems or inhibition and sorbic acids are more undissociated and thus of cell wall synthesis more effective as the medium is made more acid. • Oxidation of cellular constituents Conversely, alkaline preservatives are less effec- • Hydrolysis tive in acid or neutral media and more effective in A few of the commonly used pharmaceutical alkaline media. Thus, it is meaningless to suggest preservatives and their probable modes of action preservative effectiveness at specifi c concentra- are presented in Table 4.6. tions unless the pH of the system is mentioned and the undissociated concentration of the agent Preservative Utilization is calculated or otherwise determined. Also, if formulative materials interfere with the solubility Suitable substances may be added to a pharma- or availability of the preservative agent, its chem- ceutical preparation to enhance its permanency or ical concentration may be misleading, because it usefulness. Such additives are suitable only if they may not be a true measure of the effective con- are nontoxic and harmless in the amounts admin- centration. Many incompatible combinations of istered and do not interfere with the therapeutic preservative agents and other pharmaceutical effi cacy or tests or assays of the preparation. Cer- adjuncts have been discovered in recent years, tain intravenous preparations given in large vol- and undoubtedly many more will be uncovered umes as blood replenishers or as nutrients are not in the future as new preservatives, pharmaceuti- permitted to contain bacteriostatic additives, cal adjuncts, and therapeutic agents are combined because the amounts required to preserve such for the fi rst time. Many of the recognized incom- large volumes would constitute a health hazard patible combinations that inactivate the preserva- when administered to the patient. Thus prepara- tive contain macromolecules, including various tions like dextrose injection, USP, and others com- cellulose derivatives, polyethylene glycols, and monly given as fl uid and nutrient replenishers by natural gums. These include tragacanth, which intravenous injections in amounts of 500 to can attract and hold preservative agents, such as 1,000 mL may not contain antibacterial preserva- the parabens and phenolic compounds, rendering tives. On the other hand, injectable preparations



PRESERVATIVE PROBABLE MODES OF ACTION Benzoic acid, boric acid, p-hydroxybenzoates Denaturation of Phenols and chlorinated phenolic compounds Lytic and denaturation action on cytoplasmic membranes and for chlorinated preservatives, also by oxidation of enzymes Alcohols Lytic and denaturation action on membranes Quaternary compounds Lytic action on membranes Mercurials Denaturation of enzymes by combining with thiol (-SH) groups)

given in small volumes—for example, morphine varies with the pH, dissociation, and other sulfate injection, USP, which provides a therapeutic factors already indicated as well with the amount of morphine sulfate in approximately a presence of other formulative ingredients with 1-mL volume—can be preserved with a suitable inherent preservative capabilities. preservative without the danger of the patient For each type of preparation to be preserved, receiving an excessive amount of the preservative. the research pharmacist must consider the infl u- Examples of the preservatives and their con- ence of the preservative on the comfort of the centrations commonly employed in pharmaceu- patient. For instance, a preservative in an oph- tical preparations are benzoic acid (0.1% to thalmic preparation must have an extremely low 0.2%), sodium benzoate (0.1% to 0.2%), alcohol degree of irritant qualities, which is characteris- (15% to 20%), phenylmercuric nitrate and ace- tic of chlorobutanol, benzalkonium chloride, and tate (0.002% to 0.01%), phenol (0.1% to 0.5%), phenylmercuric nitrate, frequently used in oph- cresol (0.1% to 0.5%), chlorobutanol (0.5%), thalmic preparations. In all instances, the pre- benzalkonium chloride (0.002% to 0.01%), and served preparation must be biologically tested to combinations of methylparaben and propylpara- determine its safety and effi cacy and shelf-tested ben (0.1% to 0.2%), the latter being especially to determine its stability for the intended shelf good against fungus. The required proportion life of the product.


GROUP ACTIVITIES INDIVIDUAL ACTIVITIES 1. Develop a listing of examples where patients 1. Given a specifi c dosage form, list the signs of misunderstand the intent of the administra- degradation a pharmacist might observe indi- tion of a pharmaceutical dosage form. cating product instability. 2. Develop a listing of examples where patients 2. Given a concentration of drug in a liquid dos- misuse/abuse a pharmaceutical dosage form. age form, determine its type of degradation 3. Explain the appropriate use of specifi c dosage rate and calculate its half life and when its forms for different patient types, e.g., geriatric, concentration will be 90% of the labeled pediatric, visually impaired, hearing impaired. amount. 4. Identify four ophthalmic products with differ- 3. Compare and contrast a zero-order rate of ing preservative agents and provide a rationale degradation and a fi rst-order rate of degrada- for the selection of the specifi c preservative in tion. the product. 4. Make a listing of drugs that follow a zero-order 5. Identify elixir dosage form products that con- rate of degradation in a liquid dosage form. tain minimal or no alcohol content. Explain 5. Make a listing of drugs that follow fi rst-order the reasons for this misnomer. rates of degradation in a liquid dosage form


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